Add basic UEFI boot procedure manpage

Reviewed by:	brueffer
MFC after:	4 days
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D959
Do nothing in vt_upgrade if there is no vt driver

Previously, if no drivers attached at boot we would panic with
"vtbuf_fill_locked begin.tp_row 0 must be < screen height 0".

PR:		192248
Reviewed by:	ray
MFC after:	3 days
Sponsored by:	DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D954