Deduplicate NRF52 startOTAUpdate()

The startOTAUpdate() is the same for all NRF52 boards. Use a common
implementation for all boards that currently have a specific
implementation.

The following boards currently have an empty startOTAUpdate() for
whatever reasons and therefore are not inheriting NRF52BoardOTA to
keep the same state: Nano G2 Ultra, Seeed SenseCAP T1000-E,
Wio WM1110.

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

variants/rak4631/RAK4631Board.h

@@ -29,8 +29,9 @@
 #define  PIN_VBAT_READ    5
 #define  ADC_MULTIPLIER   (3 * 1.73 * 1.187 * 1000)
 
-class RAK4631Board : public NRF52Board {
+class RAK4631Board : public NRF52BoardOTA {
 public:
+  RAK4631Board() : NRF52BoardOTA("RAK4631_OTA") {}
   void begin();
 
   #define BATTERY_SAMPLES 8
@@ -50,6 +51,4 @@ public:
   const char* getManufacturerName() const override {
     return "RAK 4631";
   }
-
-  bool startOTAUpdate(const char* id, char reply[]) override;
 };