Redesign CLI

[pont-us]

xcengine's functionality has grown significantly since the initial version and the CLI command structure is starting to look like a poor fit. It probably needs a redesign -- not yet, but when the functionality has stabilized a bit more.

Current considerations:

• xcetool should perhaps be renamed to xcengine.
• Top-level subcommands build and run would make more sense than the current image and make-script.
  • Promote build and run from image subcommands to top-level subcommands.
  • Distribute current make-script functionality among the new build and run subcommands.

These considerations may change as functionality evolves.

See also #43 .