kernel: mtdsplit_fit: support FIT with external data

U-Boot has recently added support for having data blobs in uImage.FIT
images stored at offsets after the FDT structure rather than embedding
the data into the FDT structure itself. This is useful as it allows
parts of the image to be mapped by the FIT partition parser, and it
allows the FIT structure itself to be parsed more easily as it usually
fits into single page.
mtdsplit_fit assumed that the total length of an image is identical
to the length of the FDT structure. For uImage.FIT with external data
this assumption no longer holds true.
Add support for uImage.FIT with external data to mtdsplit_fit and in
in that case only split-off rootfs_data -- selecting and mapping rootfs
partition is left to the block partition parser just like on UBI and
block/GPT based platforms.

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