roms: List and describe the Makefile 'clean' rule

Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190506141923.12183-3-philmd@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

roms/Makefile

@@ -62,6 +62,8 @@ default:
 	@echo "  u-boot.e500    -- update u-boot.e500"
 	@echo "  u-boot.sam460  -- update u-boot.sam460"
 	@echo "  efi            -- update UEFI (edk2) platform firmware"
+	@echo "  clean          -- delete the files generated by the previous" \
+	                          "build targets"
 
 bios: build-seabios-config-seabios-128k build-seabios-config-seabios-256k
 	cp seabios/builds/seabios-128k/bios.bin ../pc-bios/bios.bin