sanitycheck: Allow to pass command with argument to ser_pty

By default, subprocess.Popen commands are supplied as a list of strings.
Using split() allows to use command with arguments, for example it is
possible to use following:

sanitycheck ... \
--device-serial-pty="set_ambient read_terminal.py" \
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>