loader: Add loader.exit

Add loader.exit(status). While one can get alomst this behavior with
loader.perform("quit"), quit doesn't allow a value to be returned to the
firmware. The interpretation of 'status' is firmware specific. This can
be used when autobooting doesn't work in scripts, for example, to allow
the firmware to try something else...

Sponsored by: Netflix

Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44094