kits: buildsys build-kit command

[webern]

Description

In order to build a kit image, buildsys will need to be invoked from a kit's build.rs file with the command buildsys build-kit. This is a new subcommand for buildsys

Definition of Done

For the sake of making smaller, incremental PRs, it is Ok if this is not fully functional at first. It will be made fully functional in https://github.com/bottlerocket-os/twoliter/issues/17

This issue is done when some semblance of the command exists in Buildsys.

[webern]

Closing as duplicate of #17