Add an option (-O) to calculate rootfs offset for combined images.
This is needed for the TP-Link mtdsplit driver to locate the rootfs
when the start is not aligned to an erase block. This will be the
case for sysupgrade images produced by tplink-safeloader with upcoming
dynamic partition splitting.
Signed-off-by: Alex Maclean <monkeh@monkeh.net>