Only create a tmpfs big enough for DIB_MIN_TMPFS

This will help to ensure images that would cause bug #1289582 would
still fail even on systems that have more RAM. Also this will ensure we
only reserve enough tmpfs space for this build.

Change-Id: Icece64e691d126a5fc89262897630fdbef1ffdf1
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 "Only create a tmpfs big enough for DIB_MIN_TMPFS"
Merge "serial-console: Use udev rules to startup getty"