Fix romstage creation with gcc 4.6 and CAR targets

Fix romstage creation with gcc 4.6 and CAR targets

newer gcc versions generate ".section .text" instead of just ".text"
in their assembler output. This patch makes sure that we don't end up
with a superfluous ".section" that makes the build fail.

Add -Wno-unused-but-set-variable to CFLAGS if the flag exists.

Change-Id: I7f24c987433cc5886dde2af27498d3331cbda303
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/252
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>