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 use doxygen as a source browser and want
a to have a quicker html output.

Tested on libdrm:
GENERATE_DOCS_FOR = default values
  Elapsed (wall clock) time (h:mm:ss or m:ss): 0:14.27
  9285 files in html dir

GENERATE_DOCS_FOR = <>
  Elapsed (wall clock) time (h:mm:ss or m:ss): 0:02.65
  717 files in html dir

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>