Fixes and improvements

[CristiFati]

• 7981f16:
  • If a model with an unsupported platform is encountered, an error test is displayed but the conversion is still attempted with no module loaded. Didn't actually try (it should fail), but it looks incorrect
• 39d26ae:
  • model_path is a wrong (and misleading) name
  • model_file_path field contains the model file name (relative to the .yml file). But when the script is invoked like python rknn_convert.py path/to/model_config.yml ..., path/to is being prepended to the relative one, resulting in an invalid path
• 45ffba3:
  • A bit of refactoring around reading config
  • Small variable renames and text changes
• 3c8c1cf:
  • Extract common code
  • Moving around, so that similar platforms come consecutively
• d929cea:
  • Field name:
    • Not consistent with the others (- instead of _)
    • Not very suggestive (although that comes from the rknn package)
  • Field value:
    • Is a list of lists of ints. No reason to store it as a string then parse it
• 3aa5419 + a345d99:
  • Properly parse and handle script arguments