binman: Support splitting an ELF file into multiple nodes

Some boards need to load an ELF file using the 'loadables' property, but
the file has segments at different memory addresses. This means that it
cannot be supplied as a flat binary.

Allow generating a separate node in the FIT for each segment in the ELF,
with a different load address for each.

Also add checks that the fit,xxx directives are valid.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>