loader: Add a "kernel.loaded" hook

This hook can be useful, for example to run a local function to choose
different modules to load when a user has picked a different kernel
from the menu.

Reviewed by: kevans
Approved by: mav (mentor)
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D24115