extract-cert: Cope with multiple X.509 certificates in a single file

This is not required for the module signing key, although it doesn't do any
harm — it just means that any additional certs in the PEM file are also
trusted by the kernel.

But it does allow us to use the extract-cert tool for processing the extra
certs from CONFIG_SYSTEM_TRUSTED_KEYS, instead of that horrid awk|base64
hack.

Also cope with being invoked with no input file, creating an empty output
file as a result.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>