image: use portable filename in 'append-image-stage'

The 'append-image-stage' command doesn't work when setting the
EXTRA_IMAGE_NAME option of the ImageBuilder as in that case
DEVICE_IMG_PREFIX is modified and no longer matches the value it had in
buildroot. Choose a filename independent of DEVICE_IMG_PREFIX for
images staged using 'append-image-stage' to fix that.

Fixes: de4b29d ("image: introduce 'append-image-stage' build command")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

include/image-commands.mk

@@ -33,11 +33,11 @@ endef
 
 ifdef IB
 define Build/append-image-stage
-	dd if=$(STAGING_DIR_IMAGE)/$(DEVICE_IMG_PREFIX)-$(1) >> $@
+	dd if=$(STAGING_DIR_IMAGE)/$(BOARD)$(if $(SUBTARGET),-$(SUBTARGET))-$(DEVICE_NAME)-$(1) >> $@
 endef
 else
 define Build/append-image-stage
-	dd if=$(BIN_DIR)/$(DEVICE_IMG_PREFIX)-$(1) of=$(STAGING_DIR_IMAGE)/$(DEVICE_IMG_PREFIX)-$(1)
+	dd if=$(BIN_DIR)/$(DEVICE_IMG_PREFIX)-$(1) of=$(STAGING_DIR_IMAGE)/$(BOARD)$(if $(SUBTARGET),-$(SUBTARGET))-$(DEVICE_NAME)-$(1)
 	dd if=$(BIN_DIR)/$(DEVICE_IMG_PREFIX)-$(1) >> $@
 endef
 endif