selftests: add EXTRA_CLEAN for clean target

Some testcases need the clean extra data after running. This patch
introduce the "EXTRA_CLEAN" variable to address this requirement.

After KBUILD_OUTPUT is enabled in later patch, it will be easy to
decide to if we need do the cleanup in the KBUILD_OUTPUT path(if the
testcase ran immediately after compiled).

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/exec/Makefile

@@ -1,5 +1,14 @@
 CFLAGS = -Wall
 
+TEST_GEN_PROGS := execveat
+TEST_GEN_FILES := execveat.symlink execveat.denatured script subdir
+# Makefile is a run-time dependency, since it's accessed by the execveat test
+TEST_FILES := Makefile
+
+EXTRA_CLEAN := subdir.moved execveat.moved xxxxx*
+
+include ../lib.mk
+
 subdir:
 	mkdir -p $@
 script:
@@ -11,13 +20,3 @@ execveat.symlink: execveat
 execveat.denatured: execveat
 	cp $< $@
 	chmod -x $@
-
-TEST_GEN_PROGS := execveat
-TEST_GEN_FILES := execveat.symlink execveat.denatured script subdir
-# Makefile is a run-time dependency, since it's accessed by the execveat test
-TEST_FILES := Makefile
-
-include ../lib.mk
-
-clean:
-	rm -rf $(TEST_GEN_PROGS) $(TEST_GEN_FILES) subdir.moved execveat.moved xxxxx*