bcm63xx: Move the OpenWrt rootfs length field

Dual image capable CFEs store an image sequence at the same place as
currently OpenWrt stores the actual rootfs length, so it will get
overwritten when flashing through such a CFE.

To prevent this from happening, move the rootfs length field to the next
four bytes, thus completely using the reserved1 field.

Since the reserved1 field is now completely in use, it does not make sense
to allow it to be set from the imagetag utility, so remove the option.

Signed-off-by: Jonas Gorski <jonas.gorski+openwrt@gmail.com>
SVN-Revision: 26680