doc: Edit application code development document.

Also - fixed variable usage and ref links.
Also - fix trailing whitespace and missed deletion of Postrequisites section.

Change-Id: I72f857c8a82048922265db99669263050d1542ff
Signed-off-by: L.S. Cook <leonax.cook@intel.com>
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>