move all build into folder _build

examples/make.mk

@@ -27,7 +27,7 @@ else
 endif
 
 # Build directory
-BUILD = build-$(BOARD)
+BUILD = _build/build-$(BOARD)
 
 # Board specific
 include $(TOP)/hw/bsp/$(BOARD)/board.mk

examples/rules.mk

@@ -100,7 +100,7 @@ size: $(BUILD)/$(BOARD)-firmware.elf
 	-@echo ''
 
 clean:
-	rm -rf build-$(BOARD)
+	rm -rf $(BUILD)
 	
 # Flash binary using Jlink
 ifeq ($(OS),Windows_NT)