config knob to control what docs to generate

The added config option, GENERATE_DOCS_FOR, lists the
stages we want to generate docs for.

It helps if you want to use doxygen as a source browser
only (like OpenGrok,DXR).

One unexpected behaviour I noticed is when I remove 'examples' from
GENERATE_DOCS_FOR.
It doesn't get rid of the examples but also skips over file docs,
so the file links in files.html, will be invalid. The icon links will
still point to the colored sources.

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>