package/systemd: check BR2_TARGET_GENERIC_GETTY

Do not create a [serial-]getty link if BR2_TARGET_GENERIC_GETTY is not set.
Might be useful for custom setups (rootfs overlay).

[Peter: use BR2_TARGET_GENERIC_GETTY_PORT to match the code below]
Signed-off-by: André Erdmann <dywi@mailerd.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>