amd64 UEFI loader: stop copying staging area to 2M physical

On amd64, add a possibility to activate kernel with staging area in place.
Add 'copy_staging' command to control this. For now, by default the
old mode of copying kernel to 2M phys is retained. It is going to be
changed in several weeks.

On amd64, add some slop to the staging area to satisfy both requirements
of the kernel startup allocator, and to have space for minor staging data
increase after the final size is calculated. Add a new command
'staging_slop' to control its size.

Improve staging area resizing, in particular, reallocate it anew if
we cannot grow it neither down nor up.

Reviewed by: kevans, markj
Discussed with: emaste (the delivery plan)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D31121