Create boot_img as a global variable

Create boot_img as a global variable

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