Move psm-based EFI loader to be dboot-style, embedded into the unix image

[r1mikey]

Having a separate shim loader has been useful so far, but it makes the UEFI/ACPI port look unnecessarily different from i86pc.

Move this code into unix, called eboot (like my first attempt at this stuff). Along the way, simplify the implementation as much as possible, especially given the full ELF loading of the subsequent kernel won't be needed.

This will also make the ELF loading path in loader64.efi simpler. It'll be very close to the FreeBSD code, with the caveat being that we simply need to load on a 2MiB boundary (technically any page boundary will be fine, but 2MiB will help us use large pages for the kernel later).