Increase the available flash memory size in AVM Fritz!Box 3370 by
incorporating the unused extra partitions located after the ubi partition.

Note that users upgrading from a previous OpenWRT version need to
re-install from the boot loader to pick up the new partition layout.

Available flash space for rootfs+overlay increases from 48MB to 124MB.

Reverting to the OEM firmware is still possible (via the recovery utility
provided by AVM) as the OEM firmware appears to reformat the config and
nand-filesystem partitions upon first boot if necessary. The
reserved-kernel and reserved-filesystem partitions are overwritten by the
OEM firmware when installing an update, so their contents do not matter.

Boot loader and device-specific information (MAC addresses, calibration
data, etc.) are not located in NAND flash and remain unharmed by this
changed.

Tested with OEM firmware 06.54 on device with HWRevision 5 and Micron
flash chip.

Signed-off-by: Michael Kuron <m.kuron@gmx.de>