x86/EFI: sanitize build logic

x86/EFI: sanitize build logic

With changes done over time and as far as linking goes, the only special
things about building with EFI support enabled are
- the need for the dummy relocations object (for xen.gz uniformly in all
  build stages, for xen.efi in stage 1),
- the special efi/buildid.o file, which can't be made part of
  efi/built_in.o, due to the extra linker options required for it.
All other efi/*.o can be consumed from the built_in*.o files.

In efi/Makefile, besides moving relocs-dummy.o to "extra", also properly
split between obj-y and obj-bin-y.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>