keymile: Move sourcing of common Kconfig

The way board/keymile/Kconfig is written protects the options there from
being parsed on non-keymile platforms.  We cannot however safely source
this file from multiple locations.  This does not manifest as a problem
currently as there are no choice statements inside of this file (nor the
sub-Kconfig files it sources).  However, moving some target selection to
one of these files exposes the underlying problem.  Rework things so
that we have this file sourced in arch/Kconfig.

Cc: Holger Brunck <holger.brunck@hitachienergy.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Holger Brunck <holger.brunck@hitachienergy.com>