dtoc: Support reading a list of arguments

It is helpful to support a string or stringlist containing a list of
space-separated arguments, for example:

   args = "-n fred", "-a", "123";

This resolves to the list:

   -n fred -a 123

which can be passed to a program as arguments.

Add a helper to do the required processing.

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