redhat/Makefile: General improvements and fixes

Depends: !1728 (merged)

This is a collection of various improvements and fixes for the redhat/Makefile
variables. These fixes include renaming and reorganizing of variables, an
explanation of how the kernel NVR is constructed, and fixing the dist-brew &
distg-brew so they can be executed without specifying any extra variables.

Signed-off-by: Prarit Bhargava prarit@redhat.com