add the --make-parents option to mkdir()

[TonyLHansen]

By default you cannot recursively create a hierarchy of directories with one mkdir command. You may create each parent directory with separate mkdir command calls, or use the --make-parents option.

To make a directory under the root called 'code/for/ampy', along with all missing parents:

  ampy --port /board/serial/port mkdir --make-parents /code/for/ampy