Initial support for implementing the bootXXX.efi workaround

Too many version of UEFI firmware (so far only confirmed on amd64)
don't really support efibootmgr selection of boot. That's the most
reliable, when it works, since there's no guesswork. However, many do
not save, unmolested, the variables that efibootmgr sets, so as a
fallback we also install loader.efi as bootXXX.efi (where XXX is
either aa64 or x64) if it doesn't already exist in /efi/boot on the
ESP. The standard only defines this for removable devices, but it's
almost ubiquitously used as a fallback. Many BIOSes implement a drive
selection feature that takes over the efibootmgr protocol, rendinering
it useless (either generally, or for those vendors not on the short
list). bootxxx.efi works around this. However, we don't install it
unconditionally there, as that breaks some popular multi-boot setups.

MFC After: 1 week
Differential Revision: https://reviews.freebsd.org/D26428