Change default value of kern.bootfile to reflect reality

In most cases kernel.bootfile is populated from the information
provided by loader(8). There are certain scenarios when loader
is not available, for instance when kernel is loaded by u-boot
or some other BootROM directly. In this case the default value
"/kernel" points to invalid location and breaks some functinality,
like using installkernel on self-hosted system or dtrace's CTF
lookup. This can be fixed by setting the value manually but the
default that reflects correct location is better than default that
points to invalid one.

Current default was set around FreeBSD 1, when "/kernel" was the
actual path. Transition to /boot/kernel/kernel happened circa FreeBSD 3.

PR: 221550
Reviewed by: ian, imp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D18902