packages/systemd: fix double getty on console

When selecting "console" for the automatic getty, the buildroot logic
would collide with systemd's internal console detection logic, resulting
in two getty being started on the console.

This commit fixes that by doing nothing when "console" is selected and
letting systemd-getty-generator deal with starting the proper getty.

Note that if something other than the console is selected
* Things will work properly, even if the selected terminal is also the
  console
* A getty will still be started on the console.
  This is what systemd has been doing on buildroot since the beginning. it
  could be disabled but I left it for backward compatibility

Fixes: #12361
Signed-off-by: Jérémy Rosen <jeremy.rosen@smile.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>