EFI: strip xen.efi when putting it on the EFI partition

EFI: strip xen.efi when putting it on the EFI partition

With debug info retained, xen.efi can be quite large. Unlike for xen.gz
there's no intermediate step (mkelf32 there) involved which would strip
debug info kind of as a side effect. While the installing of xen.efi on
the EFI partition is an optional step (intended to be a courtesy to the
developer), adjust it also for the purpose of documenting what distros
would be expected to do during boot loader configuration (which is what
would normally put xen.efi into the EFI partition).

Model the control over stripping after Linux'es module installation,
except that the stripped executable is constructed in the build area
instead of in the destination location. This is to conserve on space
used there - EFI partitions tend to be only a few hundred Mb in size.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Tested-by: Henry Wang <Henry.Wang@arm.com>
Tested-by: Wei Chen <Wei.Chen@arm.com> # arm
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>