binman: Introduce binman, a tool for building binary images

This adds the basic code for binman, including command parsing, processing
of entries and generation of images.

So far no entry types are supported. These will be added in future commits
as examples of how to add new types.

See the README for documentation.

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