kselftest: add kselftest-clean rule

We use

    $make TARGETS="size timers" kselftest

to build and run selftests. but there is no rule
for us to clean the kselftest generated files.

This patch add the rules, for example:

	$ make TARGETS="size timers" kselftest-clean

can clean all kselftest generated files.
Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michal Marek <mmarek@suse.com>

Makefile

@@ -1075,6 +1075,9 @@ PHONY += kselftest
 kselftest:
 	$(Q)$(MAKE) -C tools/testing/selftests run_tests
 
+kselftest-clean:
+	$(Q)$(MAKE) -C tools/testing/selftests clean
+
 # ---------------------------------------------------------------------------
 # Modules
 
@@ -1282,6 +1285,7 @@ help:
 	@echo  '  kselftest       - Build and run kernel selftest (run as root)'
 	@echo  '                    Build, install, and boot kernel before'
 	@echo  '                    running kselftest on it'
+	@echo  '  kselftest-clean - Remove all generated kselftest files'
 	@echo  ''
 	@echo  'Kernel packaging:'
 	@$(MAKE) $(build)=$(package-dir) help