km: modify Kconfig file organization for KM boards

As preparation for the upcoming transferring of configurations from header
files to Kconfig, a common Kconfig file for all KM boards was created. For
the moment, it only sources the other three, more specific, Kconfig files.
Therefore, the architecture Kconfig files now include the common Kconfig
file. Also, the configuration selection for KM boards was moved from the
architecture Kconfig files to the board specific Kconfig files.

Signed-off-by: Pascal Linder <pascal.linder@edu.hefr.ch>
Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
Cc: Mario Six <mario.six@gdsys.cc>
Cc: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Signed-off-by: Stefan Roese <sr@denx.de>