libfdt: Detected out-of-space with fdt_finish()

libfdt: Detected out-of-space with fdt_finish()

At present the Python sequential-write interface can produce an error when
it calls fdt_finish(), since this needs to add a terminating tag to the
end of the struct section.

Fix this by automatically expanding the buffer if needed.

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