Add a show-targets target

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

Makefile

@@ -449,6 +449,9 @@ external-deps:
 	@$(MAKE) -Bs BR2_WGET=$(TOPDIR)/toolchain/wget-show-external-deps.sh \
 		SPIDER=--spider source
 
+show-targets:
+	@echo $(TARGETS)
+
 ifeq ($(BR2_CONFIG_CACHE),y)
 # drop configure cache if configuration is changed
 $(BUILD_DIR)/tgt-config.cache: .config