Run 1wire scan at boot

[jdavid]

Run 1wire scan at boot if the 1wire action is planned, and the onewire.txt file is missing.
To do so we have to configure the pin(s) to use, probably in config.h to make it simple.

This issue comes from the problem we had with the mote at Goulasjavri, which had a filesystem issue and the onewire.txt file was lost. The mote resumed operation some time later, but didn't use the ds18b20 chain because of the onewire.txt missing.