certs: Only allow certs signed by keys on the builtin keyring

Originally the secondary trusted keyring provided a keyring to which extra
keys may be added, provided those keys were not blacklisted and were
vouched for by a key built into the kernel or already in the secondary
trusted keyring.

On systems with the machine keyring configured, additional keys may also
be vouched for by a key on the machine keyring.

Prevent loading additional certificates directly onto the secondary
keyring, vouched for by keys on the machine keyring, yet allow these
certificates to be loaded onto other trusted keyrings.

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>