Creating partitions through script in init.rc
service myscript /system/bin/logwrapper /system/bin/myscript.sh
   disabled
   oneshot


If you declare the service as disabled, it wont run unless you later explicitly start it.

But do you really want to repartition the card every time you boot the system? I would think you would want to partition it once, and then perhaps mount the partitions in the init script.