Fix to allow .config to define WOLFTPM variable (and others).

Author: dgarske

tools/config.mk

@@ -27,10 +27,10 @@ ifeq ($(ARCH),)
   V?=0
   SPMATH?=1
   RAM_CODE?=0
-  DUALBANK_SWAP=0
+  DUALBANK_SWAP?=0
   IMAGE_HEADER_SIZE?=256
-  PKA=1
-  WOLFTPM=0
+  PKA?=1
+  WOLFTPM?=0
   WOLFBOOT_PARTITION_SIZE?=0x20000
   WOLFBOOT_SECTOR_SIZE?=0x20000
   WOLFBOOT_PARTITION_BOOT_ADDRESS?=0x20000