kernel: fix FIT partition parser compatibility issues

kernel: fix FIT partition parser compatibility issues

The uImage.FIT partition parser used to squeeze in FIT partitions in
the range where partition editor tools (fdisk and such) expect the
regular partition. This is confusing people and tools when adding
additional partitions on top of the partition used for OpenWrt's
uImage.FIT.
Instead of squeezing in the additional partitions, rather start with
all uImage.FIT partitions at offset 64.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>