allow advertising updates at runtime

[floe]

I have a usecase where I want to update the advertisement data while the system is running. A hacky solution is to simply call begin() again, but that essentially resets the SoC and also leaks memory. I've refactored the advertisement code so it can be called independently using startAdvertising().

Best, Florian

[sandeepmistry]

@floe thanks for submitting, I'll leave this open for now. I'd like to ensure it's doable and compatible on nRF8001 as well.

[floe]

OK, thanks. I don't have access to an nRF8001 at the moment, and the code looks quite ... involved ;-), so I'll leave this as-is at the moment.