Python command line builder

[shuhaowu]

Right now build.py is really just a hard coded script to build the focal RT image. We want to turn it into a proper command line utility, capable of building even outside the rootdir of this repo (for custom images).

Some immediate feature requests:

• Move qemu-user-static and pause_after configurations into the command line tool.
• Create command line shortcuts to cleanup the host system if something goes wrong
• Create command line shortcuts to chroot into the target image for testing