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/promicro/PromicroBoard.h

@@ -20,12 +20,13 @@
 #define  PIN_VBAT_READ 17
 #define  ADC_MULTIPLIER   (1.815f) // dependent on voltage divider resistors. TODO: more accurate battery tracking
 
-class PromicroBoard : public NRF52Board {
+class PromicroBoard : public NRF52BoardOTA {
 protected:
   uint8_t btn_prev_state;
   float adc_mult = ADC_MULTIPLIER;
 
 public:
+  PromicroBoard() : NRF52BoardOTA("ProMicro_OTA") {}
   void begin();
 
   #define BATTERY_SAMPLES 8
@@ -75,6 +76,4 @@ public:
   void powerOff() override {
     sd_power_system_off();
   }
-
-  bool startOTAUpdate(const char* id, char reply[]) override;
 };