modsign: add markers to endif-statements in certs/Makefile

It's a bit hard for eye to track certs/Makefile if you are not
accustomed to it. This commit adds comments to key endif statements in
order to help to keep the context while reading this file.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>