fmap.py: use argparse for parsing the command line This makes the code easier to manage and provides automatic support for things like -h/--help. BUG=chromium:982465 TEST=`./fmap.py bin/example.bin` works Change-Id: I7a45d414cb2ed2d23bf65978b9dd06e05fc99cd7 Reviewed-on: https://chromium-review.googlesource.com/1693883 Tested-by: Mike Frysinger <[email protected]> Commit-Ready: Mike Frysinger <[email protected]> Legacy-Commit-Queue: Commit Bot <[email protected]> Reviewed-by: Duncan Laurie <[email protected]>