hg: build-infra/jdk9/hotspot: Improve custom hooks, and improve comments.

[magnus.ihse.bursie at oracle.com]

Changeset: 01576e6c30db
Author:    ihse
Date:      2016-01-23 00:06 +0100
URL:       http://hg.openjdk.java.net/build-infra/jdk9/hotspot/rev/01576e6c30db

Improve custom hooks, and improve comments.

! makefiles/Dist.gmk
! makefiles/lib/JvmFeatures.gmk
! makefiles/lib/JvmMapfile.gmk