main/lk2nd: make all subpackages install the same file (MR 2638)

We should always install lk image as /boot/lk2nd.img, even for
image with appended DTB (lk2nd-appended-dtb.img). pmbootstrap tooling
will expect it this way, and also it keeps all subpackages consistent,
while also making them conflict with each other.

Add new subpackages that are explicitly named -appended, but still
install the same file.

Each device should "know" which kind of LK image it needs and should
explicitly depends on specific variant.

Having only one file `/boot/lk2nd.img` will simplify many things,
for example we don't have to introduce new deviceinfo properties
to specify which image is needed (like if we installed both of them).

Signed-off-by: Alexey Minnekhanov <alexeymin@postmarketos.org>