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.

BUG=none
TEST=manual
   . ran make with GDB_DEBUG=1 and observed the modified compiler
     invocation line in the log.

Change-Id: Ie0be653509509eeb64ea3a7229f54c0c812840a9
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/196359

Makefile.inc

@@ -208,7 +208,7 @@ INCLUDES += -Isrc/device/oprom/include
 # abspath is a workaround for romcc
 INCLUDES += -include $(src)/include/kconfig.h
 
-CFLAGS = $(INCLUDES) -Os -pipe -g -nostdinc
+CFLAGS = $(INCLUDES) -pipe -g -nostdinc
 CFLAGS += -nostdlib -Wall -Wundef -Wstrict-prototypes -Wmissing-prototypes
 CFLAGS += -Wwrite-strings -Wredundant-decls -Wno-trigraphs
 CFLAGS += -Wstrict-aliasing -Wshadow
@@ -216,6 +216,11 @@ ifeq ($(CONFIG_WARNINGS_ARE_ERRORS),y)
 CFLAGS += -Werror
 endif
 CFLAGS += -fno-common -ffreestanding -fno-builtin -fomit-frame-pointer
+ifneq ($(GDB_DEBUG),)
+CFLAGS += -O0
+else
+CFLAGS += -Os
+endif
 
 additional-dirs := $(objutil)/cbfstool $(objutil)/romcc $(objutil)/ifdtool $(objutil)/options