make: Add .config file processing (v3)

In this file one can add options with which to build CRIU.
Each line is (for now) expanded into CONFIG_$(TEXT) macros
defined in config.h that can be tested later in the code.

v2: Add .config to .gitignore
v3: Don't check that make mrproper removes .config

https://github.com/xemul/criu/issues/235
travis-ci: success for Don't compile in binfmt_misc dumping code by default (rev3)
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Reviewed-by: Dmirty Safonov <dsafonov@virtuozzo.com>

.gitignore

+.config
 *.o
 *.d
 *.a

Makefile.config

@@ -14,6 +14,11 @@ endif
 
 export LIBS += $(LIBS_FEATURES)
 
+CONFIG_FILE = $(SRC_DIR)/.config
+
+$(CONFIG_FILE):
+	touch $(CONFIG_FILE)
+
 export DEFINES += $(FEATURE_DEFINES)
 export CFLAGS += $(FEATURE_DEFINES)
 
@@ -29,12 +34,13 @@ endif
 endef
 
 define config-header-rule
-$(CONFIG_HEADER): $(SRC_DIR)/scripts/feature-tests.mak
+$(CONFIG_HEADER): $(SRC_DIR)/scripts/feature-tests.mak $(CONFIG_FILE)
 	$$(call msg-gen, $$@)
 	$(Q) @echo '#ifndef __CR_CONFIG_H__'				> $$@
 	$(Q) @echo '#define __CR_CONFIG_H__'				>> $$@
 	$(Q) @echo ''							>> $$@
 $(call map,gen-feature-test,$(FEATURES_LIST))
+	$(Q) @cat $(CONFIG_FILE) | sed -n -e '/^[^#]/s/^/#define CONFIG_/p' >> $$@
 ifeq ($$(VDSO),y)
 	$(Q) @echo '#define CONFIG_VDSO'				>> $$@
 	$(Q) @echo ''							>> $$@

scripts/build/Dockerfile.tmpl

@@ -28,4 +28,4 @@ RUN make mrproper
 RUN make -j $(nproc) CC=$CC criu/parasite-syscall.o
 RUN make -j $(nproc) CC=$CC
 RUN make mrproper
-RUN bash -c 'CLEAN="$(git clean -ndx --exclude=scripts/build)"; echo "${CLEAN}"; test -z "${CLEAN}"; exit $?'
+RUN bash -c 'CLEAN="$(git clean -ndx --exclude=scripts/build --exclude=.config)"; echo "${CLEAN}"; test -z "${CLEAN}"; exit $?'