Remove NRF52BoardOTA class and integrate it into NRF52Board

As all NRF52 boards now have OTA support, let's remove the subclass
and integrate it into the base class.

Signed-off-by: Frieder Schrempf <frieder@fris.de>

src/helpers/NRF52Board.cpp

@@ -54,7 +54,7 @@ float NRF52Board::getMCUTemperature() {
   return temp * 0.25f; // Convert to *C
 }
 
-bool NRF52BoardOTA::startOTAUpdate(const char *id, char reply[]) {
+bool NRF52Board::startOTAUpdate(const char *id, char reply[]) {
   // Config the peripheral connection with maximum bandwidth
   // more SRAM required by SoftDevice
   // Note: All config***() function must be called before begin()