x86: don't build with EFI support in shim-exclusive mode

x86: don't build with EFI support in shim-exclusive mode

There's no need for xen.efi at all, and there's also no need for EFI
support in xen.gz since the shim runs in PVH mode, i.e. without any
firmware (and hence by implication also without EFI one).

The slightly odd looking use of $(space) is to ensure the new ifneq()
evaluates consistently between "build" and "install" invocations of
make.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>