added list-defconfigs and %_defconfig

support/config/config.mk

@@ -0,0 +1,19 @@
+define list-defconfigs
+	@first=true; \
+	for defconfig in $(1)/configs/*_defconfig; do \
+		[ -f "$${defconfig}" ] || continue; \
+		if $${first}; then \
+			if [ "$(2)" ]; then \
+				printf 'External configs in "$(call qstrip,$(2))":\n'; \
+			else \
+				printf "Built-in configs:\n"; \
+			fi; \
+			first=false; \
+		fi; \
+		defconfig="$${defconfig##*/}"; \
+		printf "  %-35s - Build for %s\n" "$${defconfig}" "$${defconfig%_defconfig}"; \
+	done; \
+	$${first} || printf "\n"
+endef
+
+.PHONY: list-defconfigs