With a patch to assemble-image.sh, this change to the user interface
enables a copy of the root file system to be built into a UBIFS image
and stored on the SD card for use in flashing systems with the same root
It is possible to make this a checkbox that would show up when sdimg is
selected, but that would complicate the logic quite a bit. There seems
to be a reasonable amount of space on the line, so the simpler logic
seemed like the better trade-off.