Get the information from the image that we're booting and store it in
a global variable. Prefer using this to passing it around. Remove the
special case for zfs that set the preferred boot handle by having it
uses this global variable diretly.
Reviewed by: kevans@
Differential Revision: https://reviews.freebsd.org/D20015
This update does add diag and debug capabilities to interpret the efi
variables, configuration and protocols (lsefi).
The side effect is that we add/update bunch of related headers.
