Makefile: Explain process of handling CONFIG_* variables.

Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1721 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1