This is a workaround for using other bank overlays with fitblk
partitions.

With fitblk, overlay has no constant flash location and changes
if boot or rootfs size change.
With this patch, using data from upgrade image, we calculate other
overlay offset, mount it as a loopback device and use as ovelay.

This is a temporary solution until we use uimage.fit and OpenWrt
fstools.

Signed-off-by: Andreas Gnau <andreas.gnau@iopsys.eu>