binman: Add support for generating a FIT

binman: Add support for generating a FIT

FIT (Flat Image Tree) is the main image format used by U-Boot. In some
cases scripts are used to create FITs within the U-Boot build system. This
is not ideal for various reasons:

- Each architecture has its own slightly different script
- There are no tests
- Some are written in shell, some in Python

To help address this, add support for FIT generation to binman. This works
by putting the FIT source directly in the binman definition, with the
ability to adjust parameters, etc. The contents of each FIT image come
from sub-entries of the image, as is normal with binman.

Signed-off-by: Simon Glass <sjg@chromium.org>