kernel: allow skipping deprecated uImage.FIT partition parser

In order to allow gradually migrating the boards currently using the
uImage.FIT partition (deprecated/rejected) parser to the new fitblk
driver, skip the partition parser code in case the new fitblk driver
(which serves the same purpose) is used.

As an indicator for the use of the new fitblk driver, check if the
/chosen/rootdisk property is present in the Device Tree.

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