efi: Move common EFI stub code from x86 arch code to common location
No code changes made, just moving functions and #define from x86 arch
directory to common location. Code is shared using #include, similar
to how decompression code is shared among architectures.
Signed-off-by: Roy Franz <roy.franz@linaro.org> Acked-by: Mark Salter <msalter@redhat.com> Reviewed-by: Grant Likely <grant.likely@linaro.org> Signed-off-by: Matt Fleming <matt.fleming@intel.com>