util/crossgcc/buildgcc: Use pre-set CROSSGCC_VERSION if possible

For reproducibility, a version string is appended to the version of the
tools used in the cross-toolchain. Currently, git is used to determine
that version string at runtime of this script. There are cases, where
it's not possible to determine that version string, e.g. when a release
tarball is used, and if so, the version string is just `v_`.

Thus, allow pre-setting the variable `CROSSGCC_VERSION`.

Change-Id: I888ccd877c93436b5e033528c43bd8667b8d2f10
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58396
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>

util/crossgcc/buildgcc

@@ -16,7 +16,9 @@
 
 cd "$(dirname "$0")" || exit 1
 
+if [ -z "$CROSSGCC_VERSION" ]; then
 	CROSSGCC_VERSION="$(git log -n 1 --pretty=%cd --date=short .)_$(git log -n 1 --pretty=%h .)"
+fi
 
 # default settings
 PACKAGE=GCC