packages: Remove the /boot hack from mtree-to-plist.awk

Currently, files in /boot (other than /boot/kernel) are assigned to the
bootloader package using a filename match in mtree-to-plist.awk. This
causes some problems, most notably that debug info for userboot ends up
in the utilities-dbg package instead of bootloader-dbg.

Remove the path handling from mtree-to-plist and instead set PACKAGE
in the appropriate Makefiles to put these in the correct package.

While here, move userboot*.so from bootloader-dev to bootloader.

MFC after: 3 days
Reviewed by: cperciva
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53179