only clean generated files with make clean

* this is the standard behavior

Author: sashimi-yzh

Makefile

@@ -16,16 +16,13 @@ compile:
 bsp:
 mill -i mill.bsp.BSP/install
 
-clean:
-git clean -fd
-
-cleanAll:
-git clean -fdx
-
 reformat:
 mill -i __.reformat
 
 checkformat:
 mill -i __.checkFormat
 
+clean:
+-rm -rf $(BUILD_DIR)
+
 .PHONY: test verilog help compile bsp reformat checkformat clean