mips: boot: add support for self-extracting FIT images (vmlinuz.itb)

Commit c3e2ee65 ("MIPS: generic: Add support for zboot") added
support for self-extracting images to Generic MIPS. However, the
intended way to boot Generic MIPS kernels is using FIT Images and
UHI boot protocol, but currently there's no way to make self-extracting
FIT Image (only legacy uzImages).
Add a target for this named "vmlinuz.itb", which will consist of
vmlinuz.bin and selected DT blobs. It will allow to have the advantages
of both UHI and self-extracting images.

Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>