In commit 4944aa48 ("fix(ble): Update to changes from new SDK")
a call to `BbDrvDisable()` was added to `sleep_deepsleep()`.  This
function must, however, only be called when BLE was previously
initialized, otherwise a wakeup from deepsleep will not be possible (if
it ever reaches it?).

Fix this by reworking the BLE enabled check to also be usable here, to
only call `BbDrvDisable()` when BLE is active.

Fixes: 4944aa48 ("fix(ble): Update to changes from new SDK")
Fixes: #231

We have encountered a device where the connection from the PMIC's ADMUX
pin to the MCUs ADC is severed and thus the low battery check would
trigger immediately.

For making such devices still usable, add a config option to disable the
low battery check entirely.

Fixes: #227

This fixes a potential system panic if there is already an interrupt pending