certs: Move load_certificate_list() to be with the asymmetric keys code

Move load_certificate_list(), which loads a series of binary X.509
certificates from a blob and inserts them as keys into a keyring, to be
with the asymmetric keys code that it drives.

This makes it easier to add FIPS selftest code in which we need to load up
a private keyring for the tests to use.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Herbert Xu <herbert@gondor.apana.org.au>
cc: keyrings@vger.kernel.org
cc: linux-crypto@vger.kernel.org
Link: https://lore.kernel.org/r/165515742145.1554877.13488098107542537203.stgit@warthog.procyon.org.uk/