Makefile: drop config cache on .config changes

The config cache might contain invalid values when .config is changed
(E.G. when adding/removing libraries), so drop it so config checks are
rerun for new packages (existing package still won't be rebuilt).

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

Makefile

@@ -440,6 +440,14 @@ external-deps:
 	@$(MAKE) -Bs BR2_WGET=$(TOPDIR)/toolchain/wget-show-external-deps.sh \
 		SPIDER=--spider source
 
+ifeq ($(BR2_CONFIG_CACHE),y)
+$(BUILD_DIR)/tgt-config.cache: .config
+	rm -f $@
+	touch $@
+
+$(BASE_TARGETS): | $(BUILD_DIR)/tgt-config.cache
+endif
+
 else # ifeq ($(BR2_HAVE_DOT_CONFIG),y)
 
 all: menuconfig