image: introduce 'append-image-stage' build command

image: introduce 'append-image-stage' build command

Similar to 'append-image' this new command appends an existing binary.
'append-image-stage' also makes a copy of that binary and keeps it in
$(STAGING_DIR_IMAGE). When called from within the ImageBuilder, this
copy is used instead of expecting the binary to be present.
This is useful for artifacts which include the initramfs/recovery image
which is usually not included in the ImageBuilder.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>