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.
Signed-off-by: Dmitry Eremin-Solenikov <dmitry_eremin-solenikov@mentor.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>