convert license headers to SPDX

Includes license review of 3rd-party files, adding missing upstream
headers, and moving the UNSW OZPLB license into its own file rather
than replicating it in the header.

elfloader: improve EFI support

Rather than using a bunch of assembly code to relocate the
ELF loader, we use gnu-efi[1] to build an EFI executable.
This approach handles things like nested structs better, by including
a "proper" way to handle ELF relocations.

[1]: https://sourceforge.net/projects/gnu-efi/