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

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: de4b29dab9 ("image: introduce 'append-image-stage' build command")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>