@last_good_known snapshot always points to last snapshot generated
   - points to the "date" name

handle imx6/rpi differences
transfer u-boot to image as well for imx6
fix for roots being not umounted on exit (blocking following calls to
this tool)
using skinny-metadata by default on fs creation
introduction od "update all" first list menu entry (when selected all
listed updates will be installed)
verup