Makefile: Add .config target

.config is a file in build/$BOARD that contains all defined config
variables. It is used by external programs to know what features
have been compiled for a specific board.

BUG=chrome-os-partner:33908
TEST=Check .config is generated properly. Check content, check
it updates when new .bin are created.
BRANCH=none

Change-Id: If93c2a393867a9fcb60841e282d8e6b933f3dfec
Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/233888
Reviewed-by: Vincent Palatin <vpalatin@chromium.org>

Makefile

@@ -30,6 +30,9 @@ PROJECT?=ec
 # Output directory for build objects
 out?=build/$(BOARD)
 
+# File containing configuration information
+config=$(out)/.config
+
 # If no key file is provided, use the default dev key
 PEM ?= board/$(BOARD)/dev_key.pem
 
@@ -136,7 +139,10 @@ all-y+=$(call objs_from_dir,test,$(PROJECT))
 dirs=core/$(CORE) chip/$(CHIP) board/$(BOARD) private common power test util
 dirs+=$(shell find driver -type d)
 
-all: $(out)/$(PROJECT).bin utils ${PROJECT_EXTRA}
+$(config): $(out)/$(PROJECT).bin
+	@printf '%s=y\n' $(_tsk_cfg) $(_flag_cfg) > $@
+
+all: $(config) utils ${PROJECT_EXTRA}
 
 include Makefile.rules