Update the systemd serial port element implementation
so that it uses udev rules. Much nicer than
hard coding ttyS0 and ttyS1.
The udev rule only adds real serial device types
(i.e. == 4). See:
define TTY_DRIVER_TYPE_SERIAL 0x0003
from include/linux/tty_driver.h.
Change-Id: I5d7de4815fd09c01693b7da4b8949f22fee20745
