menuselect: Various menuselect enhancements

* Add 'external' as a support level.
* Add ability for module directories to add entries to the menu
  by adding members to the <module_prefix>/<module_prefix>.xml file.
* Expand the description field to 3 lines in the ncurses implementation.
* Allow the description field to wrap in the newt implementation.
* Add description field to the gtk implementation.

Change-Id: I7f9600a1984a42ce0696db574c1051bc9ad7c808