HEADS UP: Standalone kernel debug files moving out of /boot/kernel/

I am preparing to move the standalone kernel debug data out of
/boot/kernel/ into /usr/lib/debug/boot/kernel/, mirroring the approach
used for userland debug data. This significantly reduces the boot
partition size requirement, and is a step towards supporting the
installation of kernel debug data ony when required. LLDB and GDB
automatically search for debug data under /usr/lib/debug/ so this
change should be transparent from an end-user perspective.

The change can be reviewed in Phabricator at
https://reviews.freebsd.org/D1006 and can be fetched as a unified diff
from https://people.freebsd.org/~emaste/patches/D1006.diff

This does not change any defaults or knobs: kernel debug files are
still built by default, and may be disabled by setting
WITHOUT_KERNEL_SYMBOLS=YES in /etc/src.conf. I hope to rationalize
this with userland debug in a later step.

Note that the change renames the intermediate and debug data files to
be consistent with userland debug data: in the build directory the
kernel with debug data included is now named kernel.full, and and
kernel.debug is the standalone debug data file.

I plan to merge this in a few days if there are no issues reported in
further review or testing.

-Ed