modules: use configurable poll interval

Make poll interval configurable. Use option from

----
config core 'main'
    option pollinterval '5'
---

This will only be used if the interval option is less or equal zero.
If the poll interval is not configured then a default value of 5 seconds
is used.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>