Add support for overlayfs backed on persistent device (e.g. USB key)

[rmetrich]

• Bumped new micro release
• Modified 'overlayroot into 'overlayroot=' where ='tmpfs' or a real device (can also be specified by tag)
• Fixed a typo in 'overlayroot-motd.sh'
• Added code to resynchronize the lower on boot when a 'overlayroot.resync' file is present in upper