Need way to inject network knowledge at config time

[terrillmoore]

#343 points out that you have to add network-specific knowledge in the device to enable advanced features like 250 kHz or FSK. We don't have a good way to do this at the moment. other than to put the knowledge in the arduino-lorawan library. It might be good to have library subroutines for post-join that do the setup (so that users don't have to rewrite this code every time). Users should be able to just say "I'm using TTN" and the library should configure the LMIC appropriately.

In addition, I've heard rumors of networks that do not like devices to ADR to higher spreading factors than SF10, even though SF12 is defined for that region. (This seems like something an operator might be concerned about, to manage bandwidth, so it doesn't seem crazy.) But this means we need limits that are operator-specific.