redhat: use centos x509.genkey file if building under centos
Upstream Status: RHEL only
When doing modinfo against a kernel built under centos, it'll show the signer as Red Hat Enterprise Linux kernel. To avoid this, switch the module signing to use a centos x509.genkey file provided by Brian Stinson.
This was requested for centos-stream-9 in BZ 2029952.
v2: fix build issue due empty if/then clause
Signed-off-by: Herton R. Krzesinski herton@redhat.com