MODSIGN: Use utf8 strings in signer's name in autogenerated X.509 certs

Place an indication that the certificate should use utf8 strings into the
x509.genkey template generated by kernel/Makefile.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>