called from main select loop in mavproxy when the pppd child
        sends us some data

def ppp_read(self, ppp_fd):
        '''called from main select loop in mavproxy when the pppd child
        sends us some data'''
        buf = os.read(ppp_fd, 100)
        if len(buf) == 0:
            # EOF on the child fd
            self.stop_ppp_link()
            return
        print("ppp packet len=%u" % len(buf))
        master = self.master
        master.mav.ppp_send(len(buf), buf)