AI 147053: Add -hdf primer vars for the platform and release ID associated with the...

AI 147053: Add -hdf primer vars for the platform and release ID associated with the current (most recent) SDK.
  Currently, the docs that are specific to each SDK release -- downloading, installing, upgrading, requirements -- are stored in a directory with release-derived name, such as "1.1_r1". This change lets documents use generic links to point into the most recent version specific docs, eg href="{@docRoot}sdk/{sdkCurrent}/installing.html" instead of href="{@docRoot}sdk/1.1_r1/installing.html".
  BUG=1790234

Automated import of CL 147053