image: add gzip-filename build recipe

Some vendors use basic gzip metadata (original filename and timestamp)
to verify valid images, along with the size of it's contents.

Also, add a new device profile variable FACTORY_IMG_NAME
which would be ideal to use with this new recipe.

Signed-off-by: Michael Pratt <mcpratt@pm.me>

include/image-commands.mk

@@ -316,6 +316,15 @@ define Build/gzip
 	@mv $@.new $@
 endef
 
+define Build/gzip-filename
+	@mkdir -p $@.tmp
+	@cp $@ $@.tmp/$(word 1,$(1))
+	$(if $(SOURCE_DATE_EPOCH),touch -hcd "@$(SOURCE_DATE_EPOCH)" $@.tmp/$(word 1,$(1)) $(word 2,$(1)))
+	$(STAGING_DIR_HOST)/bin/gzip -f -9 -N -c $@.tmp/$(word 1,$(1)) $(word 2,$(1)) > $@.new
+	@mv $@.new $@
+	@rm -rf $@.tmp
+endef
+
 define Build/install-dtb
 	$(call locked, \
 		$(foreach dts,$(DEVICE_DTS), \