EFI: don't repeatedly replace symlinks

EFI: don't repeatedly replace symlinks

Once created there's no point re-creating them on every incremental
make. This in particular prevents them from becoming root-owned during
e.g. "sudo make install-xen", but it also allows (during development)
to replace them there (instead of in common/efi/) by actual files with
perhaps slightly changed contents.

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