Makefile tweaks. (#376)

-Have fusee-secondary clean sept-secondary as required.
-Use $(TARGET) variable for sept-secondary, and clean the encrypted output as required.

fusee/fusee-secondary/Makefile

@@ -159,6 +159,7 @@ clean:
 	@$(MAKE) -C $(AMS)/exosphere clean
 	@$(MAKE) -C $(AMS)/thermosphere clean
 	@$(MAKE) -C $(AMS)/stratosphere clean
+	@$(MAKE) -C $(AMS)/sept clean
 	@rm -fr $(BUILD) $(TARGET).bin $(TARGET).elf
 
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