libpayload: 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 '-g -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 V=1 GDB_DEBUG=1 and observed the modified compiler
      invocation line in the log.

Change-Id: I4a902ad25157fa7f93614917f440e56145405a1d
Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/201783
Reviewed-by: David Hendricks <dhendrix@chromium.org>
Reviewed-by: Trevor Bourget <tbourget@codeaurora.org>

payloads/libpayload/Makefile.inc

@@ -53,12 +53,18 @@ subdirs-$(CONFIG_LP_CBFS) += libcbfs
 subdirs-$(CONFIG_LP_LZMA) += liblzma
 
 INCLUDES := -Iinclude -Iinclude/$(ARCHDIR-y) -I$(obj)
-CFLAGS =  $(EXTRA_CFLAGS) $(INCLUDES) -Os -pipe -nostdinc -ggdb3
+CFLAGS =  $(EXTRA_CFLAGS) $(INCLUDES) -pipe -nostdinc -ggdb3
 CFLAGS += -nostdlib -fno-builtin -ffreestanding -fomit-frame-pointer
 CFLAGS += -Wall -Wundef -Wstrict-prototypes -Wmissing-prototypes
 CFLAGS += -Wwrite-strings -Wredundant-decls -Wno-trigraphs
 CFLAGS += -Wstrict-aliasing -Wshadow -Werror
 
+ifneq ($(GDB_DEBUG),)
+CFLAGS += -O0 -g
+else
+CFLAGS += -Os
+endif
+
 $(obj)/libpayload-config.h: $(KCONFIG_AUTOHEADER)
 	cmp $@ $< 2>/dev/null || cp $< $@