chroot/other/kernel_flavor_installed: support generic kernel name

kernel is named /boot/vmlinuz now, looking at the filename will no
longer tell us what flavor it is. This now will look at
/usr/share/kernel, which has always contained the kernel 'flavor', and
since we currently only install 1 kernel these days, guarding this with
pmaports.cfg should be unnecessary. In the worst case (if there are
multiple kernel 'flavors' installed), it'll just grab the first one and
return it.