EFI: move x86 boot/runtime code to common/efi

EFI: move x86 boot/runtime code to common/efi

This moves the EFI boot and runtime services code to the common/efi directory.
This code is symbolicly linked back into the arch/x86/efi directory where it is
built if a build-time check for PE/COFF support in the toolchain passes.  In
the PE/COFF supporting case, both the EFI executable and the normal Xen image
(with stubbed EFI functions) are built.  We can't use the normal common build
infrastructure since we are building two versions at the same time, with
different EFI related code in each.  No code changes, just file movement and
make updates.  The files are symbolicly linked at build time back toe the
original arch/x86/efi directory.  This is in preparation for adding ARM EFI
support where much of these files can be shared.

Signed-off-by: Roy Franz <roy.franz@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>