MIPS: Support generating Flattened Image Trees (.itb)

Add support for generating kernel images in the Flattened Image Tree
(.itb) format as supported by U-Boot. This format is essentially a
Flattened Device Tree binary containing images (kernels, DTBs, ramdisks)
and configurations which link those images together. The big advantages
of FIT images over the uImage format are:

  - We can include FDTs in the kernel image in a way that the bootloader
    can extract it & manipulate it before providing it to the kernel.
    Thus we can ship FDTs as part of the kernel giving us the advantages
    of being able to develop & maintain the DT within the kernel tree,
    but also have the benefits of the bootloader being able to
    manipulate the FDT. Example uses for this would be to inject the
    kernel command line into the chosen node, or to fill in the correct
    memory size.

  - We can include multiple configurations in a single kernel image.
    This means that a single FIT image can, given appropriate
    bootloaders, be booted on different boards with the bootloader
    selecting an appropriate configuration & providing the correct FDT
    to the kernel.

  - We can support a multitude of hashes over the data.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/14352/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>