certs: simplify empty certs creation in certs/Makefile

To create an empty cert file, we need to pass "" to the extract-cert
tool, which is common for all the three call-sites of cmd_extract_certs.

Factor out the logic into extract-cert-in.

One exceptional case is PKCS#11 case, where we override extract-cert-in
with the URI.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>