terminal.py: Fix Xfce on ubuntu/debian; some cleanup

terminal.py: Fix Xfce on ubuntu/debian; some cleanup

    * Xfce class was setting and passing wrong variable for ubuntu/debian.
    * Xfce class was using -e instead of -x for passing command.  The former creates
      a shell escape nightmare
    * Clean up local and instance/class variables with same name but different usage.
    * Remove side-effect and directly return formatted command for clarity.

(From OE-Core rev: b2ee5c5e34cdc3d65ca7b5da3486360a74d6c500)

Signed-off-by: Jeffrey C Honig <jeffrey.honig@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>