Import firmware compiler flags

Makefile

 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 export FIRMWARE_ARCH
+export FIRMWARE_CONFIG_PATH
 
 export CC ?= gcc
 export CXX ?= g++
-ifeq ($(FIRMWARE_ARCH),)
-export CFLAGS = -Wall -Werror -DCHROMEOS_ENVIRONMENT
-else
-export CFLAGS = -Wall -Werror
+export CFLAGS
+
+# Include compiler flags if provided.
+# The CC and CFLAGS should not be overridden hereafter.
+ifneq ($(FIRMWARE_CONFIG_PATH),)
+include $(FIRMWARE_CONFIG_PATH)
 endif
 
+# Provides default optimization level if not set by FIRMWARE_CONFIG_PATH
 ifeq (${DEBUG},)
-CFLAGS += -O3
+CFLAGS ?= -O3
 else
-CFLAGS += -O0 -g -DVBOOT_DEBUG
+CFLAGS ?= -O0
+endif
+
+ifeq ($(FIRMWARE_ARCH),)
+CFLAGS += -Wall -Werror -DCHROMEOS_ENVIRONMENT
+else
+CFLAGS += -Wall -Werror
+endif
+
+ifneq (${DEBUG},)
+CFLAGS += -g -DVBOOT_DEBUG
 endif
 
 ifeq (${DISABLE_NDEBUG},)
 CFLAGS += -DNDEBUG
 endif
 
 export TOP = $(shell pwd)
 export FWDIR=$(TOP)/firmware
 export HOSTDIR=$(TOP)/host
 ifeq ($(FIRMWARE_ARCH),)
@@ skipping 31 matching lines @@
 
 install:
         $(MAKE) -C utility install
         $(MAKE) -C cgpt install
 
 runtests:
         $(MAKE) -C tests runtests
 
 rbtest:
         $(MAKE) -C tests rbtest