binman: Make the tooldir configurable

binman: Make the tooldir configurable

Add a command-line argument for setting the tooldir, so that the default
can be overridden. Add this directory to the toolpath automatically.
Create the directory if it does not already exist.

Put the default in the argument parser instead of the class, so that it
is more obvious.

Update a few tests that expect the utility name to be provided without
any path (e.g. 'futility'), so they can accept a path, e.g.
/path/to/futility

Update the documentation and add a few tests.

Improve the help for --toolpath while we are here.

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