systemd-serialtty: allow empty package

systemd-serialtty: allow empty package

If SERIAL_CONSOLES is set to empty for some reason, we would have rootfs
failure like below.

  - nothing provides systemd-serialgetty needed by systemd-1:234-r0.core2_64

This is because systemd-serialgetty package is not generated when
SERIAL_CONSOLES is empty.

Set ALLOW_EMPTY to "1" for this recipe to allow for generation of empty
systemd-serialgetty package to avoid the above error.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>