sandbox: Return '-c command' exit value as sandbox exit code

When a command is passed into sandbox using the '-c' argument the
command is run directly. This is most helpful when running tests (such
as test-dm.sh). Previously the exit code was an unused enum. Change it
to be the actual return code from the command so that the script calling
sandbox can know if the command succeeded (tests passed).  Also remove
the now completely unused "exit_state" in sandbox.

Signed-off-by: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Simon Glass <sjg@chromium.org>