move channel_num to LoRaConfig

[andrekir]

channel_num defines the LoRa channel frequency.

• one single lora.channel_num users can choose for the LoRa frequency (and QR codes);
• simplifies channel logic, making primary/secondary channel roles/index/order irrelevant.

note: when unset device still picks a random channel_num based on the primary channel hash (no change).

not a breaking change.

[garthvh]

Makes more sense this way

[caveman99]

for the next release (1.3.42) there's a proto version increase in the code already, so the user config will be wiped anyway. i say yank it now, don't deprecate it.

[andrekir]

reindexing channel.proto will break all previous channels url/qr codes, clients/app won't connect to any previous versions, and we'll need to reversion the url to https://www.meshtastic.org/f/#blablabla

I'd rather not turn this into a breaking change as it is mostly cosmetic, we can tag deprecated to breaking proto changes and schedule "one last breaking change" yank-fest later, if everyone agrees.

[garthvh]

I agree with this approach

[geeksville]

This pull request has been mentioned on Meshtastic. There might be relevant details there:

https://meshtastic.discourse.group/t/manual-channel-number-not-available-in-1-3/6635/6