Add DFU suffix to .bin file

See https://community.spark.io/t/dfu-util-version-0-8-coming-soon/6408/15
Move dfu-suffix call to program-dfu target

Also fix dfu-suffix call with vendor and product flags,
not device flag, which was incorrect.
Merge pull request #323 from spark/feature/dfu-suffix

Add DFU suffix to .bin file