treewide: simplify inclusion of subtarget image files

treewide: simplify inclusion of subtarget image files

Many target use a repetitive if-include scheme for their subtarget
image files, though their names are consistent with the subtarget
names.

This patch removes these redundant conditions and just uses the
variable for the include where the target setup allows it.

For sunxi, this includes a trivial rename of the subtarget image
Makefiles.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>