The self-relocation code is not efi-specific, move it to boot/common.

The function was defined as taking 4 parameters and returning EFI_STATUS,
but all existing callers (in asm code) passed only two parameters and don't
use the return value. The function signature now matches that usage, and
doesn't refer to efi-specific types.

Parameters and variables now use the cannonical typenames set up by elf.h
(Elf_Word, Elf_Addr, etc) instead of raw C types. Hopefully this will
prevent suprises as new platforms come along and use this code.

The function was renamed from _reloc() to self_reloc() to emphasize its
difference from the other elf relocation code found in boot/common.

Differential Revision: https://reviews.freebsd.org/D2490