Workaround to disable bluetooth on NRF52

[todd-herbert]

Not a fix, but a workaround for #2885. Implements a suggestion discussed on the Meshtastic Discord server:

If config.bluetooth.enabled is false, Bluefruit is setup as normal, then advertising is stopped, and the tx power is reduced to minimum. This means that the SoftDevice, and the brown-out-detection, are enabled as usual.

With this workaround, and Bluetooth disabled:

• the device does not show in scans
• it is not possible to connect to the device using Bluetooth

This workaround seems to avoid the issue of #2885, and RadioLib -706 errors (discussed in #NRF52 on Discord). I have not experienced any lock-ups or crashes during testing, even with uptime > 3 days. It would be good to get more test time on this before rolling out.

I don't have equipment to measure the power consumption over time, but crude current measurement with a digital multimeter suggests that the workaround is similar to running with Bluetooth completely uninitialized. The workaround is at most a few perfect more efficient than running normally, with Bluetooth at idle.

[thebentern]

Looks good. I want to hold this one till the release after this upcoming alpha though, just in case we encounter lockups on stop advertising.

[mike6789]

I will test this soon with power monitoring equipment, hope it fixes my issue