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>
 kselftest:
        $(Q)$(MAKE) -C tools/testing/selftests run_tests
 
+kselftest-clean:
+       $(Q)$(MAKE) -C tools/testing/selftests clean
+
 # ---------------------------------------------------------------------------
 # Modules
 
        @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