Makefile: update location of target-specific .conf
Makefile: update location of target-specific .conf

arduino_101.conf was moved up a level, but the toplevel Makefile
was not updated so that it can be located.

Signed-off-by: Andrew Boie <aab045b28152628be6af0e556050d0381fd0a34d@intel.com>

utzig/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,tamban01/mcuboot,tamban01/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,tamban01/mcuboot