Provide a way to compile some files with -O0 option

When preparing an image for source level debugging, it is convenient
to be able to compile some modules with -O0, which makes it much
easier to follow the execution flow.

This patch allows to do it by defining GDB_DEBUG=1 in the environment
before invoking make. Adding this feature as a common config flag is
problematic, because we don't want to compile the entire image with
-O0.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/196359
(cherry picked from commit dde4928c045d12e502cb109015a710cd9fdf2a04)

Changed from CFLAGS to CFLAGS_common.

Change-Id: Ie0be653509509eeb64ea3a7229f54c0c812840a9
Signed-off-by: Isaac Christensen <isaac.christensen@se-eng.com>
Reviewed-on: http://review.coreboot.org/7005
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>