Add driver for writing into the internal flash.

Write the OK flag for mcuboot using this driver.

src/drivers/InternalFlash.cpp

@@ -0,0 +1,39 @@
+#include <sdk/modules/nrfx/mdk/nrf.h>
+#include "InternalFlash.h"
+using namespace Pinetime::Drivers;
+
+void InternalFlash::ErasePage(uint32_t address) {
+  // Enable erase.
+  NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Een;
+  __ISB();
+  __DSB();
+
+  // Erase the page
+  NRF_NVMC->ERASEPAGE = address;
+  Wait();
+
+  // Disable erase
+  NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren;
+  __ISB();
+  __DSB();
+}
+
+void InternalFlash::WriteWord(uint32_t address, uint32_t value) {
+  // Enable write.
+  NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Wen;
+  __ISB();
+  __DSB();
+
+  // Write word
+  *(uint32_t*)address = value;
+  Wait();
+
+  // Disable write
+  NRF_NVMC->CONFIG = NVMC_CONFIG_WEN_Ren;
+  __ISB();
+  __DSB();
+}
+
+void InternalFlash::Wait() {
+  while (NRF_NVMC->READY == NVMC_READY_READY_Busy) {;}
+}

src/drivers/InternalFlash.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <cstdint>
+
+namespace Pinetime {
+  namespace Drivers {
+    class InternalFlash {
+    public:
+      static void ErasePage(uint32_t address);
+      static void WriteWord(uint32_t address, uint32_t value);
+    private:
+      static inline void Wait();
+    };
+  }
+}