Makefile: Fix inclusion of the config-devices.mak.d Kconfig dependencies

The Makefile tries to include device Kconfig dependencies via

 -include $(SUBDIR_DEVICES_MAK_DEP)

and thus expects files that match *-softmmu/config-devices.mak.d ...
however, the minikconf script currently generates files a la
"*-softmmu-config.devices.mak.d" instead, so the dependency files
simply got ignored so far. For example, after a "touch hw/arm/Kconfig",
the arm-softmmu/config-devices.mak file is currently not re-generated.
Fix it by putting the dependency files in the *-softmmu folders now.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>