Install serial-getty@ units under getty.target

Install serial-getty@ units under getty.target

Having serial-getty@ symlinks in sysinit target doesn't make much
sense as it not only requires udev to create respective device nodes
but it also causes following ordering cycle:

  systemd[1]: Found ordering cycle on basic.target/start
  systemd[1]: Walked on cycle path to sysinit.target/start
  systemd[1]: Walked on cycle path to serial-getty@ttyS0.service/start
  systemd[1]: Walked on cycle path to basic.target/start
  systemd[1]: Breaking ordering cycle by deleting job serial-getty@ttyS0.service/start

This commit moves getty symlinks back where these belong.

Change-Id: Ie14177d3dae049e809c668a2c43306c4361a1a7b