MFC r293233: loader.efi: add terminal emulation support

This is based on the vidconsole implementation.

Submitted by: Toomas Soome <tsoome@me.com>
Relnotes: Yes

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Add a version of the FreeBSD bootloader which can run in userland, packaged
as a shared library. This is intended to be used by BHyVe to load FreeBSD
kernels into new virtual machines.