When the rootfs wants us to perform an upgrade from USB, a valid .conf file is already available on the USB stick. So we need a way to tell the rescue system: a reflash was requested, but do it from the stick. This is what the USB_UPDATE=1 option is good for.
implements #13708
Rename flag as discussed.
Signal rootfs to reset configuration when flash was forced.
@chhitz, @mtross : this goes along with the other rootfs update MRs and is required for the whole thing to function.