ppc4xx: Update CPCI405 variants handling

This patch replaces the BOARD_REVISION variable in include/config.mk
by a using a temporary include file in the platform directory.

The former way does not work anymore and the latter is also used by
some other boards.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Signed-off-by: Stefan Roese <sr@denx.de>