modsign: hide openssl output in silent builds

When a user calls 'make -s', we can assume they don't want to
see any output except for warnings and errors, but instead
they see this for a warning free build:

 ###
 ### Now generating an X.509 key pair to be used for signing modules.
 ###
 ### If this takes a long time, you might wish to run rngd in the
 ### background to keep the supply of entropy topped up.  It
 ### needs to be run as root, and uses a hardware random
 ### number generator if one is available.
 ###
 Generating a 4096 bit RSA private key
 .................................................................................................................................................................................................................................++
 ..............................................................................................................................++
 writing new private key to 'certs/signing_key.pem'
 -----
 ###
 ### Key pair generated.
 ###

The out...