kernel-install/60-ukify: also support the convention with 'devicetree' file

kernel-install/60-ukify: also support the convention with 'devicetree' file

Requested in https://github.com/systemd/systemd/pull/28582#issuecomment-1673300596.
The is the last requested changed, so fixes #28771.

90-loaderentry.install is modified to also check $KERNEL_INSTALL_CONF_ROOT
when looking for the devicetree file. For normal use this is probably not
needed, but it's nice to be consistent and it also makes it much easier to
write the tests.

In tests, also do 'ukify inspect' now that we have it.