Makefile: Do not use wildcard hw/*/Kconfig as input for minikconf

The hw/*/Kconfig files should be sourced from hw/Kconfig, so there
is no need to pass them along as input files to minikconfig. We should
use the hw/*/Kconfig wildcard only for build dependencies in the Makefile.

With this change, there are now no duplicate entries in the generated
*-softmmu/config-devices.mak.d files anymore, and there is finally a
chance to get rid of stale Kconfig files like hw/bt/Kconfig, too (once
they do not show up in the config-devices.mak.d files now anymore).

Message-Id: <20200203153905.20544-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>