modsign: Use single PEM file for autogenerated key

The current rule for generating signing_key.priv and signing_key.x509 is
a classic example of a bad rule which has a tendency to break parallel
make. When invoked to create *either* target, it generates the other
target as a side-effect that make didn't predict.

So let's switch to using a single file signing_key.pem which contains
both key and certificate. That matches what we do in the case of an
external key specified by CONFIG_MODULE_SIG_KEY anyway, so it's also
slightly cleaner.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/.gitignore b/.gitignore
index 4ad4a98..17fa24d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -97,6 +97,7 @@
 # Leavings from module signing
 #
 extra_certificates
+signing_key.pem
 signing_key.priv
 signing_key.x509
 x509.genkey