scripts: convert-overrides: Allow command-line customizations

Adds argument parsing to the conversion script so that the fields that
the script uses to do conversions can be customized on the command line.
The intention is to allows easier customization without having to fork
the script, and allow automated checking on 3rd party layers via CI
without false positives

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>