Fix goals for NRF51_DK makefile:

Now "all" depend on the project merged with the soft device,
the goal merged has been renamed into the file it generate and now depend
on the realization of $(PROJECT).hex

tools/export/gcc_arm_nrf51_dk.tmpl

@@ -1,7 +1,7 @@
 {% extends "gcc_arm_common.tmpl" %}
 
 {% block target_all %}
-all: $(PROJECT).bin $(PROJECT).hex size merge
+all: $(PROJECT).bin $(PROJECT)-combined.hex size
 {% endblock %}
 
 {% block additional_variables %}
@@ -13,6 +13,6 @@ SREC_CAT = srec_cat
 {% endblock %}
 
 {% block additional_targets %}
-merge:
+$(PROJECT)-combined.hex: $(PROJECT).hex
 	$(SREC_CAT) $(SOFTDEVICE) -intel $(PROJECT).hex -intel -o $(PROJECT)-combined.hex -intel --line-length=44
 {% endblock %}