Implement uefi_ignore_boot_mgr env variable.

When set, we ignore all the hints that the UEFI boot manager has set
for us. We also always fail back to the OK prompt when we can't find
the right thing to boot rather than failing back to the UEFI boot
manager. This has the side effect of also expanding the cases where we
fail back to the OK prompt to include when we're booted under UEFI,
but UEFI::BootCurrent isn't set in the environment and we can't find a
proper place to boot from.

Reviewed by: bcran
Differential Revision: https://reviews.freebsd.org/D20016