package/xserver_xorg-server: don't install init script if nodm is enabled

Both S40xorg and S90nodm tries to run an Xserver on vt1, causing the nodm
one to fail.  If nodm is enabled, then that is likely what the user wants to
run, so skip installing S40xorg.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>