Merge patch series "Provide a class for building filesystem images"

Simon Glass <sjg@chromium.org> says:

Create a class around mk_fs() to handle the common tasks of image
creation, such as managing scratch directories and cleaning up.

Start with a few small cleanups to mk_fs(), then convert the helper
to use a class.

Link: https://lore.kernel.org/r/20260309151307.831537-1-sjg@chromium.org