Add a makefile to document the release building process.

Add a makefile to document the release building process.
Specifically, this documents the available targets and relevant
environment variables for "make release".  LOCAL_PATCHES,
RELEASEDISTFILES, RELEASENOUPDATE, etc.. are covered.

A future commit should add more information about drivers.conf,
boot_crunch.conf, and other less well-known aspects of the release
build.

Reviewed and history section added by:	phk