x86/Makefile: remove $(guard) use from $(TARGET).efi target

x86/Makefile: remove $(guard) use from $(TARGET).efi target

Following the patch 65d104984c04 ("x86: fix race to build
arch/x86/efi/relocs-dummy.o"), the error message
  nm: 'efi/relocs-dummy.o': No such file"
started to appear on system which can't build the .efi target. This is
because relocs-dummy.o isn't built anymore.
The error is printed by the evaluation of VIRT_BASE and ALT_BASE which
aren't use anyway.

But, we don't need that file as we don't want to build `$(TARGET).efi'
anyway.  On such system, $(guard) evaluate to the shell builtin ':',
which prevent any of the shell commands in `$(TARGET).efi' from been
executed.

Even if $(guard) is evaluated opon use, it depends on $(XEN_BUILD_PE)
which is evaluated at the assignment. So, we can replace $(guard) in
$(TARGET).efi by having two different rules depending on
$(XEN_BUILD_PE) instead.

The change with this patch is that none of the dependency of
$(TARGET).efi will be built if the linker doesn't support PE
and VIRT_BASE and ALT_BASE don't get evaluated anymore, so nm will not
complain about the missing relocs-dummy.o file anymore.

Since prelink-efi.o isn't built on system that can't build
$(TARGET).efi anymore, we can remove the $(guard) variable everywhere.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>