image: add 'append-image' build command

Commit 7ce1d9ce09 ("build: artifacts add dependency for built images")
now makes sure that sysupgrade and initramfs images are available at
the stage that artifacts are created.
Allow making use of that with a new build command 'append-image' to
be used in artifacts.
See the next commit for an example.

Signed-off-by: Daniel Golle <[email protected]>

include/image-commands.mk

@@ -27,6 +27,10 @@ define Build/append-kernel
 	dd if=$(IMAGE_KERNEL) >> $@
 endef
 
+define Build/append-image
+	dd if=$(BIN_DIR)/$(IMG_PREFIX)$(if $(PROFILE_SANITIZED),-$(PROFILE_SANITIZED))-$(1) >> $@
+endef
+
 compat_version=$(if $(DEVICE_COMPAT_VERSION),$(DEVICE_COMPAT_VERSION),1.0)
 json_quote=$(subst ','\'',$(subst ",\",$(1)))
 #")')