Revert "Unify boot1 with loader" change r324646

Back out the unification commit to boot1. There's some issues on the
arm and arm64 platforms that need to be addressed with code
changes. There's also a discussion on arch@ about the future of
boot1.efi vs just using loader.efi that needs to play out. So take a
pause on these changes until the arm issues can be fixed and it's
clear boot1.efi will survive into FreeBSD 12.

OK'd by: Eric@meatspace