modsign: Use extract-cert to process CONFIG_SYSTEM_TRUSTED_KEYS

Fix up the dependencies somewhat too, while we're at it.

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

diff --cc kernel/Makefile
Simple merge

diff --cc kernel/system_certificates.S
Simple merge

diff --cc scripts/Makefile
Simple merge