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/Makefile b/Makefile
index 531dd16..6ab99d8 100644
--- a/Makefile
+++ b/Makefile
@@ -1173,8 +1173,8 @@
 		  arch/*/include/generated .tmp_objdiff
 MRPROPER_FILES += .config .config.old .version .old_version \
 		  Module.symvers tags TAGS cscope* GPATH GTAGS GRTAGS GSYMS \
-		  signing_key.priv signing_key.x509 x509.genkey		\
-		  extra_certificates signing_key.x509.keyid		\
+		  signing_key.pem signing_key.priv signing_key.x509	\
+		  x509.genkey extra_certificates signing_key.x509.keyid	\
 		  signing_key.x509.signer vmlinux-gdb.py
 
 # clean - Delete most, but leave enough to build external modules