uboot-imx

tools/ifdtool: Support writing multiple files (-w) simultaneously

Currently ifdtool only supports writing one file (-w) at a time.
This looks verbose when generating u-boot.rom for x86 targets.
This change allows at most 16 files to be written simultaneously.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>