lantiq: use BT HomeHub 5 Type A OEM partition layout

lantiq: use BT HomeHub 5 Type A OEM partition layout

This way the on nand bad block table is preserved and used. Add support
for nand OOB ECC checksums as well. It should fix all reported ubi
errors, which were all related to bad nand blocks and a purged on nand
bad block table.

The existing ubi partition will be reused, which eliminates the need
to touch the caldata during initial install. The BT u-boot has support
for loading a kernel from an ubi volume. It isn't necessary any longer
to replace the BT u-boot with a custom compiled one to use LEDE.

It is required to restore the BT Firmware and install LEDE from scratch
to switch to the new partition layout.

An image for restoring the BT firmware and installing LEDE is provided
at https://github.com/mkresin/lede/releases.

Signed-off-by: Mathias Kresin <dev@kresin.me>