treewide: remove implicit SUBTARGET

treewide: remove implicit SUBTARGET

Historically it's possible to leave the `SUBTARGETS` undefined and
automatically fallback to a "generic" subtarget. This however breaks
various downstream scripts which may have expectations around filenames:

While some targets with an explicit generic subtarget contain `generic`
in the filenames of artifacts, implicit "subtargets" don't.

Right now this breaks the CI[1], possibly also scripts using the ImageBuilders.

This commit removes all code that support implicit handling of
subtargets and instead requires every target to define "SUBTARGETS".

[1]: https://github.com/openwrt/openwrt/actions/runs/8592821105/job/23548273630

Signed-off-by: Paul Spooren <mail@aparcar.org>