Use the kern.bootfile sysctl to set the default kernel path rather than
hardcoding /boot/kernel. This allows pmcstat(8) to work without -k when
using nextboot -k or 'boot foo' at the loader to boot alternate kernels.

Differential Revision: https://reviews.freebsd.org/D2425
Reviewed by: adrian, emaste, gnn
MFC after: 2 weeks
Sponsored by: Norse Corp, Inc.