KEYS: Load *.x509 files into kernel keyring

Load all the files matching the pattern "*.x509" that are to be found in kernel
base source dir and base build dir into the module signing keyring.

The "extra_certificates" file is then redundant.

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

diff --cc kernel/Makefile
Simple merge

diff --cc kernel/modsign_certificate.S
Simple merge