lantiq: handle the dual-firmware layout of brnboot

brnboot based devices can have two Image partitions. When flashing
images via the brnboot recovery web interface, the Image partitions are
written alternating.

The current active Image partition is stored in the first byte of the
Primary_Setting partition by using 0x00 for Code_Image_0 and 0x01 for
Code_Image_1.

By using the information about the active "Code Image", it is possible
to ensure that the rootfs belongs to the current booted Image/Kernel.

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

SVN-Revision: 49281