Introduce support for host config cache

We now have for quite some time a configuration cache used to speed-up
the execution of ./configure scripts when compiling programs for the
target. This commit introduces a similar concept when Buildroot
compiles programs for the host.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Makefile

@@ -441,12 +441,12 @@ show-targets:
 	@echo $(TARGETS)
 
 ifeq ($(BR2_CONFIG_CACHE),y)
-# drop configure cache if configuration is changed
-$(BUILD_DIR)/tgt-config.cache: $(CONFIG_DIR)/.config
+# drop configure caches if configuration is changed
+$(BUILD_DIR)/tgt-config.cache $(BUILD_DIR)/host-config.cache: $(CONFIG_DIR)/.config
 	rm -f $@
 	touch $@
 
-$(BASE_TARGETS): | $(BUILD_DIR)/tgt-config.cache
+$(BASE_TARGETS): | $(BUILD_DIR)/tgt-config.cache $(BUILD_DIR)/host-config.cache
 endif
 
 else # ifeq ($(BR2_HAVE_DOT_CONFIG),y)
@@ -580,7 +580,7 @@ endif
 	rm -rf $(CONFIG_DIR)/.config $(CONFIG_DIR)/.config.old $(CONFIG_DIR)/.config.cmd $(CONFIG_DIR)/.auto.deps
 
 flush:
-	rm -f $(BUILD_DIR)/tgt-config.cache
+	rm -f $(BUILD_DIR)/tgt-config.cache $(BUILD_DIR)/host-config.cache
 
 %_defconfig: $(TOPDIR)/configs/%_defconfig
 	cp $^ $(CONFIG_DIR)/.config

package/Makefile.autotools.in

@@ -117,6 +117,7 @@ define $(2)_CONFIGURE_CMDS
 		LDFLAGS="$$(HOST_LDFLAGS)" \
                 $$($$(PKG)_CONF_ENV) \
 		./configure \
+		$$(if $$(filter YES,$$($$(PKG)_USE_CONFIG_CACHE)),--cache-file="$(BUILD_DIR)/host-config.cache",) \
 		--prefix="$$(HOST_DIR)/usr" \
 		--sysconfdir="$$(HOST_DIR)/etc" \
 		$$($$(PKG)_CONF_OPT) \