make: remove dependency from flash to all

* print message when no hex file is found instead
* add objcopy for native so that flash can succeed

Makefile.include

@@ -178,7 +178,16 @@ distclean:
 	-@for i in $(USEPKG) ; do "$(MAKE)" -C $(RIOTBASE)/pkg/$$i distclean ; done
 	-@rm -rf $(BINDIRBASE)
 
-flash: all
+flash: $(filter all all-%, $(MAKECMDGOALS))
+	@if [ ! -f $${HEXFILE} ]; then \
+		echo ""; \
+		echo ""; \
+		echo "You need to create a hex file before you can flash it."; \
+		echo "Unless you know otherwise, 'make all' usually does what you want."; \
+		echo ""; \
+		echo ""; \
+		exit 1; \
+	fi
 	$(FLASHER) $(FFLAGS)
 
 term:

boards/native/Makefile.include

@@ -16,7 +16,7 @@ export AR ?= $(PREFIX)ar
 export AS ?= $(PREFIX)as
 export LINK ?= $(PREFIX)gcc
 export SIZE ?= $(PREFIX)size
-export OBJCOPY = true
+export OBJCOPY = $(PREFIX)objcopy
 
 export DEBUGGER = gdb
 export TERMPROG = $(ELF)