x86: adjust EFI-related build message

x86: adjust EFI-related build message

As of commit 93249f7fc17c ("x86/efi: split compiler vs linker support"),
EFI support in xen.gz may be available even if no xen.efi gets
generated. Distinguish the cases when emitting the message.

Also drop the pointlessly (afaict) left use of $(filter ...) (needed
only when used in $(if ...)), from the ifeq() introduced by 7059afb202ff
("x86/Makefile: remove $(guard) use from $(TARGET).efi target").

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