native: added more proper command line parameters parsing

Added a basic command line parameter parsing framework

Added the following options by now:
--stop-at=<time>: Auto-stop after <time> seconds
--seed=<seed>   : random seed for entropy device
--testargs      : any argument that follows is ignored in top level
                  and made available thru
                  native_get_test_cmd_line_args()

All command line parameters are still avaliable by calling
native_get_cmd_line_args(), but now you can also call
native_get_test_cmd_line_args() to get whatever was set after
--testargs

Signed-off-by: Alberto Escolar Piedras <alpi@oticon.com>