make build: always include selectedConfiguration.mk if it exists
make build: always include selectedConfiguration.mk if it exists

This way $(DISTORTOS_CONFIGURATION_MK) variable (with path to selected
configuration file) is always available, even for simple targets. If the
file doesn't yet exist default "distortosConfiguration.mk" is used.
Otherwise the value is read from selectedConfiguration.mk.
CezaryGapinski/distortos,jasmin-j/distortos,DISTORTEC/distortos,DISTORTEC/distortos,jasmin-j/distortos,CezaryGapinski/distortos,CezaryGapinski/distortos,CezaryGapinski/distortos,DISTORTEC/distortos,jasmin-j/distortos,jasmin-j/distortos,DISTORTEC/distortos,jasmin-j/distortos,CezaryGapinski/distortos