MFC r293422 (partial):

Update generated EFI boot image templates.

This is a partial MFC as stable/10 only has EFI boot support for amd64,
and there are no plans to change this, so the other platform images
aren't included.

Sponsored by: Multiplay

MFC r281524 by andrew@: Use MACHINE in the efi loader when it is what we mean

It may not be the same as MACHINE_CPUARCH, it just happened to be the
case the architectures this code currently supports.

Fixes build failure reported by Oliver Pinter.