make user_rf_cal_sector_set a weak symbol so it can be overridden

[dfries]

This gives the user control of where the sector is allocated if they so choose or they get the default and don't have to think about it. I ran into a problem of not being able to compile esp-link because of the duplicate symbol (they defined it as well).

[pfalcon]

Why do you think it can't be overriden? How systems without weak symbols solved this issues for decades, before weak symbols appeared?