fix bug that deletes work directory on fully persistent USB drives

Currently, on startup the /home/mindstorms/work-directory is deleted on E2b fully persistent USB-sticks (see here ). This bugfix introduces a persistency check that disallows deletion of the work-directory on fully persistent USB-sticks.

requested review from @oliver.rippel

assigned to @dmehlem

approved this merge request

mentioned in commit 5cdbc3d5

merged

LGTM !