Create gptboot.efi

This is a primary boot loader that is intended to implement the
gptboot partition selection algorithm just like we did for BIOS
booting. While the preferred method for UEFI is to use the UEFI Boot
Manager protocol, there are situations where that can't be done: some
BIOS makers interfere with the protocol in unhelpful ways, there's a
new standard for a zero variable write from the client OS, and finally
for USB drives that might be mobile between systems with multiple
partitions there needs to be a media stable way to select.

Reviewed by: tsoome, bcran
Differential Revision: https://reviews.freebsd.org/D20547