make_device_config.sh: Emit dependency file to directory where included

Placing the config-devices.mak.d file alongside the config-devices.mak
file in *-softmmu/ lead to it getting included into through
*-softmmu/Makefile in addition to ./Makefile, leading to confusion.

Instead, emit it to ./%-config-devices.mak.d, where it is included.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>