powerpc/83xx/km: cleanup tuxx1 support

This is a preparation for the upcoming kmopti2 board. This board has
also a second fpga on board which is different to the tuxx1 target. But we
want to use the same header file. So remove the config option
KM_DISABLE_APP2 and simply use the board names to distinguish the features.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>