Move efi_main into efi/loader

Move the efi_main routine out of libefi into sys/boot/efi/loader.
Since boot1 has its own efi_main routine, this effectively prevents
boot1 from linking with libefi. By moving it out, we can share code
better (though though some refactoring with boot1's efi_main and
loader.efi's efi_main is definitely in order).

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