Move 'clean' to top-level makefile

So you can type 'make clean' instead of 'make GOAL=clean'

dv/uvm/core_ibex/Makefile

@@ -75,13 +75,6 @@ run:
 
 ###############################################################################
 
-.PHONY: clean
-clean:
-	rm -f $(EXT_DIR)/riscv_core_setting.sv
-	rm -rf $(OUT-DIR)
-
-###############################################################################
-
 # This is the top-level output directory. Everything we generate goes in
 # here.
 OUT := out
@@ -92,4 +85,14 @@ OUT := out
 export OUT-DIR      := $(dir $(OUT)/)
 export METADATA-DIR := $(OUT-DIR)metadata
 
+# riscv-dv extension directory
+export EXT_DIR             := riscv_dv_extension
+
+###############################################################################
+
+.PHONY: clean
+clean:
+	rm -rf $(OUT-DIR)
+	rm -f $(EXT_DIR)/riscv_core_setting.sv
+
 ###############################################################################

dv/uvm/core_ibex/wrapper.mk

@@ -23,7 +23,6 @@ all: collect_results $(if $(filter 1,$(COV)),merge_cov,)
 # Environment variables
 
 TOOLCHAIN           := ${RISCV_TOOLCHAIN}
-EXT_DIR             := riscv_dv_extension
 
 export IBEX_ROOT      := $(realpath ../../../)
 export PRJ_DIR        := $(realpath ../../..)
@@ -33,7 +32,6 @@ export LOWRISC_IP_DIR := $(realpath ${PRJ_DIR}/vendor/lowrisc_ip)
 export dv_root := $(realpath ../../../vendor/lowrisc_ip/dv)
 export DUT_TOP := ibex_top
 
-
 ###############################################################################
 # Here we express the different build artifacts that the Makefile uses to
 # establish the dependency tree, as well as which jobs depend on which