x86/build: suppress EFI-related tool chain checks upon local $(MAKE) recursion

x86/build: suppress EFI-related tool chain checks upon local $(MAKE) recursion

The xen-syms and xen.efi linking steps are serialized only when the
intermediate note.o file is necessary. Otherwise both may run in
parallel. This in turn means that the compiler / linker invocations to
create efi/check.o / efi/check.efi may also happen twice in parallel.
Obviously it's a bad idea to have multiple producers of the same output
race with one another - every once in a while one may e.g. observe

objdump: efi/check.efi: file format not recognized

We don't need this EFI related checking to occur when producing the
intermediate symbol and relocation table objects, and we have an easy
way of suppressing it: Simply pass in "efi-y=", overriding the
assignments done in the Makefile and thus forcing the tool chain checks
to be bypassed.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>