gzip: add clean-room RFC 1951/1952 inflater + libcheck tests

New src/gzip.c implements DEFLATE (RFC 1951) plus the gzip wrapper
(RFC 1952) from the RFC text only. Single-pass inflate, no allocations:
the output buffer doubles as the LZ77 sliding window, so back-references
read from out[out_pos - distance]. Canonical Huffman decode using
counts[] / symbols[] tables, ~10x smaller code than fast lookup tables
which matters in the bootloader. CRC32 + ISIZE verified against the
gzip trailer. Gated by WOLFBOOT_GZIP.

include/gzip.h carries the public entry point plus the RFC-canonical
constants (magic bytes, CM=DEFLATE, fixed Huffman boundaries, EOB
symbol, dynamic block field widths, run-length repeat metadata, CRC32
init/final-XOR, header/trailer sizes, alphabet sizes) so future
maintainers can cross-reference the RFC sections by name instead of
chasing literal numbers.

Tests in tools/unit-tests/unit-gzip.c round-trip 6 corpora through host
gzip(1) and back through wolfBoot_gunzip (empty, short text, all-zeros,
structured text, pseudo-random, ~2 MB kernel-sized). 9 negative cases
cover bad magic, bad CM, reserved FLG bits, truncated header,
truncated DEFLATE body, CRC32 mismatch, ISIZE mismatch, output overflow,
and NULL parameters. All 15 pass under libcheck.

Author: dgarske

include/gzip.h

@@ -0,0 +1,116 @@
+/* gzip.h
+ *
+ * Native gzip decompression for wolfBoot FIT subimages.
+ *
+ * Clean-room implementation of RFC 1951 (DEFLATE) and RFC 1952 (gzip).
+ *
+ * Compile with GZIP=1.
+ *
+ *
+ * Copyright (C) 2026 wolfSSL Inc.
+ *
+ * This file is part of wolfBoot.
+ *
+ * wolfBoot is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * wolfBoot is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335, USA
+ */
+#ifndef WOLFBOOT_GZIP_H
+#define WOLFBOOT_GZIP_H
+
+#include <stdint.h>
+
+/* error codes */
+#define WOLFBOOT_GZIP_E_FORMAT     -1  /* bad magic / method / reserved bits */
+#define WOLFBOOT_GZIP_E_TRUNCATED  -2  /* input ended mid-stream */
+#define WOLFBOOT_GZIP_E_OUTPUT     -3  /* output would exceed out_max */
+#define WOLFBOOT_GZIP_E_HUFFMAN    -4  /* invalid Huffman tree / code */
+#define WOLFBOOT_GZIP_E_DISTANCE   -5  /* back-ref distance > bytes written */
+#define WOLFBOOT_GZIP_E_CRC32      -6  /* trailer CRC32 mismatch */
+#define WOLFBOOT_GZIP_E_ISIZE      -7  /* trailer ISIZE mismatch */
+#define WOLFBOOT_GZIP_E_PARAM      -8  /* invalid parameter */
+
+/* RFC 1952 gzip wrapper constants */
+#define GZIP_MAGIC_ID1            0x1FU      /* first magic byte  */
+#define GZIP_MAGIC_ID2            0x8BU      /* second magic byte */
+#define GZIP_CM_DEFLATE           8          /* CM = DEFLATE      */
+#define GZIP_HEADER_MIN_SIZE      10         /* magic+CM+FLG+MTIME+XFL+OS */
+#define GZIP_TRAILER_SIZE         8          /* CRC32 + ISIZE     */
+#define GZIP_CRC32_INIT           0xFFFFFFFFU
+#define GZIP_CRC32_FINAL_XOR      0xFFFFFFFFU
+#define GZIP_CRC32_POLY           0xEDB88320U /* IEEE 802.3 reflected */
+
+/* RFC 1952 Sec. 2.3.1 header flag bits (FLG byte) */
+#define GZIP_FLG_FTEXT            0x01
+#define GZIP_FLG_FHCRC            0x02
+#define GZIP_FLG_FEXTRA           0x04
+#define GZIP_FLG_FNAME            0x08
+#define GZIP_FLG_FCOMMENT         0x10
+#define GZIP_FLG_RESERVED         0xE0
+
+/* RFC 1951 DEFLATE - alphabet sizes */
+#define GZIP_MAX_HUFF_BITS        15    /* max Huffman code length */
+#define GZIP_CL_CODES             19    /* code-length alphabet    */
+#define GZIP_LITLEN_CODES         288   /* literal/length alphabet */
+#define GZIP_DIST_CODES           32    /* distance alphabet       */
+
+/* RFC 1951 DEFLATE - fixed Huffman boundaries (Sec. 3.2.6) */
+#define GZIP_FIXED_LIT_END_8BIT   144   /* 0..143    -> 8 bits */
+#define GZIP_FIXED_LIT_END_9BIT   256   /* 144..255  -> 9 bits */
+#define GZIP_FIXED_LIT_END_7BIT   280   /* 256..279  -> 7 bits */
+#define GZIP_FIXED_LIT_END        288   /* 280..287  -> 8 bits */
+#define GZIP_FIXED_DIST_COUNT     30    /* 0..29     -> 5 bits */
+
+/* RFC 1951 DEFLATE - alphabet bounds (Sec. 3.2.4 / 3.2.5) */
+#define GZIP_EOB_SYMBOL           256   /* end-of-block marker */
+#define GZIP_LENGTH_CODE_BASE     257   /* first length code   */
+#define GZIP_LENGTH_CODE_COUNT    29    /* 257..285            */
+#define GZIP_DIST_CODE_COUNT      30    /* 0..29               */
+
+/* RFC 1951 DEFLATE - dynamic block header (Sec. 3.2.7) */
+#define GZIP_HLIT_BITS            5     /* HLIT field width    */
+#define GZIP_HDIST_BITS           5     /* HDIST field width   */
+#define GZIP_HCLEN_BITS           4     /* HCLEN field width   */
+#define GZIP_HLIT_BASE            257   /* HLIT + 257          */
+#define GZIP_HDIST_BASE           1     /* HDIST + 1           */
+#define GZIP_HCLEN_BASE           4     /* HCLEN + 4           */
+#define GZIP_CL_LEN_BITS          3     /* code-length code is 3 bits */
+
+/* RFC 1951 DEFLATE - run-length repeat symbols (Sec. 3.2.7).
+ *   sym 16: 2 extra bits, repeat previous length 3..6 times
+ *   sym 17: 3 extra bits, repeat zero        3..10 times
+ *   sym 18: 7 extra bits, repeat zero       11..138 times
+ */
+#define GZIP_REPEAT_PREV_EXTRA    2
+#define GZIP_REPEAT_PREV_BASE     3
+#define GZIP_REPEAT_Z3_EXTRA      3
+#define GZIP_REPEAT_Z3_BASE       3
+#define GZIP_REPEAT_Z7_EXTRA      7
+#define GZIP_REPEAT_Z7_BASE       11
+
+/* Decompress a gzip stream.
+ *
+ * in       - pointer to gzip stream (RFC 1952 wrapper around RFC 1951 DEFLATE)
+ * in_len   - length of gzip stream in bytes
+ * out      - destination buffer; also used as the DEFLATE sliding window,
+ *            so the output region must be readable as well as writable.
+ * out_max  - maximum bytes that may be written to out
+ * out_len  - on success, set to the number of bytes written
+ *
+ * Returns 0 on success, negative WOLFBOOT_GZIP_E_* on error.
+ */
+int wolfBoot_gunzip(const uint8_t *in, uint32_t in_len,
+                    uint8_t *out, uint32_t out_max,
+                    uint32_t *out_len);
+
+#endif /* WOLFBOOT_GZIP_H */