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
  2399  dot jobs

GENERATE_DOCS_FOR = files
  Elapsed (wall clock) time (h:mm:ss or m:ss): 0:11.19
  7065 files in html dir
  1866 dot jobs

Signed-off-by: Adrian Negreanu <adrian.m.negreanu@intel.com>