package/systemd: fix "Timed out waiting for device /dev/console."

Buildroot built with systemd fails to open a login prompt on the
serial port when /dev/console is specified as BR2_TARGET_GENERIC_GETTY_PORT
(which is its default value):

systemd[1]: dev-console.device: Job dev-console.device/start timed out.
systemd[1]: Timed out waiting for device /dev/console.
systemd[1]: Dependency failed for Serial Getty on console.
systemd[1]: serial-getty@console.service: Job serial-getty@console.service/start failed with result 'dependency'.
systemd[1]: dev-console.device: Job dev-console.device/start failed with result 'timeout'.
systemd[1]: Reached target Login Prompts.
systemd[1]: Reached target Multi-User System.

According to this issue on Github [1], serial-getty@.service should
not be instantiated on /dev/console, console-getty@.service should
be used instead. This stems from the fact that there should be no
dependency on /dev/console.

[1] https://github.com/systemd/systemd/issues/10914



Signed-off-by: Xavier Ruppen <xruppen@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Peter: drop SERVICE variable as suggested by Yann]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>