image: extend FIT partition parser for use on eMMC/SDcard

Introduce a magic GUID_PARTITION_LINUX_FIT_GUID to designate a GPT
partition to be interpreted by the FIT partition parser.
In that way, sub-partitions for (external-data) uImage.FIT stored
directly in a partition can be split, similar like we do for devices
with raw flash storage.

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