Remove scripts/coreboot-sdk.h crossgcc mirror sources

This removes the scripts/coreboot-sdk.h crossgcc build forcing the usage
of coreboot's source mirrors, with BUILDGCC_OPTIONS='--mirror'. Notably,
the coreboot.org source archive mirror URLs have been all serving 403
Forbidden notices for many days, without any external notice apparent.
Also beneficially, BUILDGCC_OPTIONS can now be controlled from the
parent's side, as any other env var.

Author: mcinglis

scripts/coreboot-sdk.sh

@@ -61,12 +61,8 @@ else
     exit 1
 fi
 
-BUILDGCC_OPTIONS+="--mirror"
-export BUILDGCC_OPTIONS
-
 make -C coreboot \
     crossgcc-x64 \
     crossgcc-i386 \
     CPUS="$(nproc)" \
-    BUILDGCC_OPTIONS="${BUILDGCC_OPTIONS}" \
     "${@}"