selftests: lib.mk: define CLEAN macro to allow Makefiles to override clean

Define CLEAN macro to allow Makefiles to override common clean target
in lib.mk. This will help fix the following failures:

warning: overriding recipe for target 'clean'
../lib.mk:55: warning: ignoring old recipe for target 'clean'

Fixes: 88baa78d1f31 ("selftests: remove duplicated all and clean target")

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/lib.mk

@@ -51,8 +51,12 @@ endef
 emit_tests:
 	$(EMIT_TESTS)
 
-clean:
+define CLEAN
 	$(RM) -r $(TEST_GEN_PROGS) $(TEST_GEN_PROGS_EXTENDED) $(TEST_GEN_FILES) $(EXTRA_CLEAN)
+endef
+
+clean:
+	$(CLEAN)
 
 $(OUTPUT)/%:%.c
 	$(LINK.c) $^ $(LDLIBS) -o $@