systemd-serialgetty: use better default SERIAL_CONSOLE

* having /dev/ttySO fails with:
  | ln: failed to create symbolic link
  `/OE/shr-core/tmp/work/nokia900-oe-linux-gnueabi/systemd-serialgetty-1.0-r0/image/etc/systemd/system/getty.target.wants/serial-getty@/dev/ttyS0.service':
  No such file or directory

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>