mtd: bcm63xxpart: handle Broadcom partition order

The original Broadcom partition order has the root fs in front of the
kernel, which resulted in miscalculated partition sizes.
Detect when such an image is on the flash and also reorder the partitions
accordingly.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>