In order to create multiple Jenkins build pipelines for this repository,
I'm splitting it up with branches. There will be a branch for each
image:
* base/main: The base image
* cross/main: The image with a cross-compiler toolchain
* build/main: The final build image
Note _/main_ in each name. The intent here is to be able to have
different "sub-branches" for each image, so e.g. there might be a
_base/dev_.
In addition to the branches for each image, there will be branches for
the shared library code as well, named like _lib/main_. The image
branches will checkout this branch using `git worktree`.
