nRF52840 - fix flashing blue light issues

Partial Fix

Problem: On the XIAO nRF52840, if ble is on and not bonded, then the blue LED is flashing. If you then turn off the software radio, stop advertising, or you try and put the board into its lowest power state by issuing sd_power_system_off(), the blue light either keeps blinking or goes solidly on. Bluetooth is not advertising or the board is in a very low power state, but the blue led stays on. There should be a way to fix this!

Partial Fix

By setting Bluefruit.autoConnLed(false) when starting advertising, the blue led does not blink at all. I don't like loosing this indicator, but better it is not there than have it give missinformation. So, I now set this to false and blink the blue led a few times at the start of advertising.