message: fix(writer): Emit checksum event when verifying bmaps
  Due to the Blockmap.FilterStream not emitting a "checksum"
  event (as it individually verifies specified ranges), the
  flashing process would get stuck on finish.
  This emits a "checksum" event on "finish" when blockmapping,
  averting this issue.
  Change-Type: patch
diff: @@ -272,7 +272,15 @@ class ImageWriter extends EventEmitter {
  if (options.image.bmap) {
  debug('verify:bmap')
  const blockMap = BlockMap.parse(options.image.bmap)
  - pipeline.append(new BlockMap.FilterStream(blockMap))
  + const blockMapStream = new BlockMap.FilterStream(blockMap)
  + pipeline.append(blockMapStream)
  +
  + // NOTE: Because the blockMapStream checksums each range,
  + // and doesn't emit a final "checksum" event, we artificially
  + // raise one once the stream finishes
  + blockMapStream.once('finish', () => {
  + pipeline.emit('checksum', {})
  + })
  } else {
  const checksumStream = new ChecksumStream({
  algorithms: options.checksumAlgorithms
  