set signing key on connection

def cmd_signing_key(self, args):
        '''set signing key on connection'''
        if len(args) == 0:
            print("usage: signing setup passphrase")
            return
        if not self.master.mavlink20():
            print("You must be using MAVLink2 for signing")
            return
        passphrase = args[0]
        key = self.passphrase_to_key(passphrase)
        self.master.setup_signing(key, sign_outgoing=True, allow_unsigned_callback=self.allow_unsigned)
        print("Setup signing key")