Signed-off-by: Peter Tyser <ptyser@xes-inc.com>

Add a simple expr style command that will set an env variable as the result
of the command.  This allows us to do simple math in shell.  The following
operations are supported: &, |, ^, +, -, *, /.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>