This class allows to hook a post install function that would mask getty
services enabled by systemd as part of `systemd_preset_all`.

To take advantage of it, an image would:
1. inherit this class
2. define SYSTEMD_MASK_GETTY to point to a list of tty devices that the
   hook will mask the getty service for.

Common use-case: disable running getty on tty1 at boot.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>