[nrfconnect] Fix BDX transfer resumption

When OTA is interrupted in the middle of a BDX transfer
and the transfer is then re-initiated after the 5-minute
idle timeout, it may fail with the following error:

dfu_target_stream: stream_flash_buffered_write error -12

Signed-off-by: Damian Krolik <damian.krolik@nordicsemi.no>

src/platform/nrfconnect/OTAImageProcessorImpl.cpp

@@ -71,7 +71,7 @@ CHIP_ERROR OTAImageProcessorImpl::PrepareDownloadImpl()
         writer.image_id = image_id;
         writer.open     = [](int id, size_t size) { return dfu_target_init(DFU_TARGET_IMAGE_TYPE_MCUBOOT, id, size, nullptr); };
         writer.write    = [](const uint8_t * chunk, size_t chunk_size) { return dfu_target_write(chunk, chunk_size); };
-        writer.close    = [](bool success) { return dfu_target_done(success); };
+        writer.close    = [](bool success) { return success ? dfu_target_done(success) : dfu_target_reset(); };
 
         ReturnErrorOnFailure(System::MapErrorZephyr(dfu_multi_image_register_writer(&writer)));
     };

src/platform/nrfconnect/OTAImageProcessorImpl.h

@@ -56,7 +56,7 @@ class OTAImageProcessorImpl : public OTAImageProcessorInterface
     bool IsFirstImageRun() override;
     CHIP_ERROR ConfirmCurrentImage() override;
 
-private:
+protected:
     CHIP_ERROR PrepareDownloadImpl();
     CHIP_ERROR ProcessHeader(ByteSpan & aBlock);