systemd-serialgetty: use existing unit files in systemd

systemd-serialgetty: use existing unit files in systemd

Now that systemd isn't deleting the serial-getty@.service unit template
files, we can simply symlink to the files provided by systemd instead of
shipping a copy of them in this recipe.

This ensures that the getty units triggered by the systemd are identical,
be them via SERIAL_CONSOLES or the generator.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>