CI: kernel: test each target with additional changes than target/linux

CI: kernel: test each target with additional changes than target/linux

Test each target if there are additional changes than target/linux.
This is needed to do wide test with changes to kmods, include/kernel and
changes to the workflow files.

While at it also cleanup and rework the code to drop duplication.
Also drop since_last_remote_commit to better track changes.

Fixes: 04ada8bc4118 ("CI: kernel: build only changed targets")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>