xcompile: Rename internal variable CFLAGS to CFLAGS_GCC

This is in preparation of adding support for clang to xcompile.

Change-Id: I518d077f134610082b0939b1525682f2289eec34
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/10423
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
1 file changed