Enable debug flags when building natively

It is still useful to build natively with debugging, particularly when
improving test code, so add this to the compiler flags in this case.

BUG=chrome-os-partner:21115
BRANCH=pit
TEST=manual
'make DEBUG=1' in the vboot directory within the chroot. See that the test
executables are now build with debugging info and gdb has line number
information.

Change-Id: Icaedae67151883673525930e25cf8b1f30654339
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/66172

Makefile

@@ -139,6 +139,7 @@ else
 # FIRMWARE_ARCH not defined; assuming local compile.
 CC ?= gcc
 CFLAGS += -DCHROMEOS_ENVIRONMENT -Wall -Werror # HEY: always want last two?
+CFLAGS += ${DEBUG_FLAGS}
 endif
 
 ifneq (${OLDDIR},)