modsign: Use if_changed rule for extracting cert from module signing key

We couldn't use if_changed for this before, because it didn't live in
the kernel/ directory so we couldn't add it to $(targets). It was easier
just to leave it as it was.

Now it's in the certs/ directory we can use if_changed, the same as we
do for the trusted certificate list.

Aside from making things consistent, this means we don't need to depend
explicitly on the include/config/module/sig/key.h file. And we also get
to automatically do the right thing and re-extract the cert if the user
does odd things like using a relative filename and then playing silly
buggers with adding/removing that file in both the source and object
trees. We always favour the one in the object tree if it exists, and
now we'll correctly re-extract the cert when it changes. Previously we'd
*only* re-extract the cert if the config option changed, even if the
actual file we're using did change.

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