treewide: rename IMAGE_PREFIX/IMAGE_NAME to DEVICE_IMG_*

treewide: rename IMAGE_PREFIX/IMAGE_NAME to DEVICE_IMG_*

We so far had two variables IMG_PREFIX and IMAGE_PREFIX with
different content. Since these names are obviously quite
confusing, this patch renames the latter to DEVICE_IMG_PREFIX,
as it's a device-dependent variable, while IMG_PREFIX is only
(sub)target-dependent.

For consistency, also rename IMAGE_NAME to DEVICE_IMG_NAME, as
that's a device-dependent variable as well.

Cc: Paul Spooren <mail@aparcar.org>
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>