IT87: Add ability to select between chips on GIGABYTE DualBIOS boards.

Thanks to Vadim Girlin for finding out how to do that.
This is known to work on GA-MA770-UD3, GA-B75M-D3V, GA-B75N and
GA-H61M-S1 (only M_BIOS is populated).

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>
Signed-off-by: Damien Zammit <damien@zamaudio.com>
Signed-off-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>
Tested-by: Damien Zammit <damien@zamaudio.com>
Tested-by: Anton Kochkov <anton.kochkov@gmail.com>
Acked-by: Stefan Tauner <stefan.tauner@student.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1720 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1
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