Implement -P for boot loader. It's a bit easier to implement here than
in boot1, like is normally done. When a keyboard appears in the UEFI
device tree, assume -D -h, just like on a BIOS boot.
- It is unclear if an ACPI keyboard appearing in the tree means there's
- a real keyboard or not. A USB keyboard doesn't seem to appear unless
- it is really there.
Differential Revision: https://reviews.freebsd.org/D5223