serial-console: Use udev rules to startup getty

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
Merge "serial-console: Use udev rules to startup getty"