Build system updates

[toasted-nutbread]

• Some restructuring for better organization.
• Adds more variant modification actions.
• Better value safety by cloning.
• Adds inheritance support.
• Adds some optional command line arguments:
  • --all - builds all variants
  • --default - updates the manifest to the default manifest
  • --manifest <target> - updates the manifest to a specific manifest version
  • <target> <target> ... - lists specific targets to build