doc: tweak Sphinx linkcheck options

Sphinx has a builder option that we can use to check links
(invoked with '$ make linkcheck' in the /doc folder)
Add some configuration tweaks to conf.py, and update the
Makefile to also use the nitpick (-n) option to Sphinx
if this linkcheck build option is selected (does much more
checking of internal references)

Change-Id: Ib413bc8d4195c72f1a8a4c345a5a722f88fad8b8
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>