EFI: move efi-boot.h inclusion point

EFI: move efi-boot.h inclusion point

When it was introduced, it was imo placed way too high up, making it
necessary to forward-declare way too many static functions. Move it down
together with
- the efi_check_dt_boot() stub, which afaict was deliberately placed
  immediately ahead of the #include,
- blexit(), because of its use of the efi_arch_blexit() hook.
Move up get_value() and set_color() to before the inclusion so their
forward declarations can also be zapped.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>