kernel.bbclass: fix installation of modules signing certificates

If one has provided external key/certificate for modules signing, Kbuild
will skip creating signing_key.pem and will write only signing_key.x509
certificate. Thus we have to check for .x509 file existence rather than
.pem one.

(From OE-Core rev: 4972582767a3325d22a16db9a5479c2d0001964b)

Signed-off-by: Dmitry Eremin-Solenikov <dmitry_eremin-solenikov@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 2527e731eba43bd36d0ea268aca6b03155376134)
Signed-off-by: Nicolas Dechesne <nicolas.dechesne@linaro.org>
Signed-off-by: Armin Kuster <akuster808@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/kernel.bbclass

@@ -451,7 +451,7 @@ do_shared_workdir () {
 	cp .config $kerneldir/
 	mkdir -p $kerneldir/include/config
 	cp include/config/kernel.release $kerneldir/include/config/kernel.release
-	if [ -e certs/signing_key.pem ]; then
+	if [ -e certs/signing_key.x509 ]; then
 		# The signing_key.* files are stored in the certs/ dir in
 		# newer Linux kernels
 		mkdir -p $kerneldir/certs