fix(writer): Emit checksum event when verifying bmaps (#1859)
fix(writer): Emit checksum event when verifying bmaps (#1859)

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
resin-io/resin-etcher,resin-io/herostratus,resin-io/etcher,resin-io/etcher,resin-io/herostratus,resin-io/etcher,resin-io/resin-etcher,resin-io/etcher,resin-io/etcher