This adds the option --torcx_store to specify the path to a
directory containing torcx images to be baked into the OS image. A
blank string can be given instead of a path to restore the previous
behavior and leave an empty vendor store.
The default value is the default path created by build_torcx_store,
which is used when build_packages updates torcx images. This means
that the current pattern "./build_packages && ./build_image prod"
should result in a fully updated OS image with all torcx images
available in the vendor store.
a0bebcc38b