tools: Add clean targets for tools directory

Adding missing clean targets for following tools directories:

  lib/bpf
  lib/subcmd
  build

This are now cleaned via 'make -C tools clean' command.
Reported-and-Tested-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1452509693-13452-2-git-send-email-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/Makefile

@@ -127,6 +127,12 @@ liblockdep_clean:
 libapi_clean:
 	$(call descend,lib/api,clean)
 
+libbpf_clean:
+	$(call descend,lib/bpf,clean)
+
+libsubcmd_clean:
+	$(call descend,lib/subcmd,clean)
+
 perf_clean:
 	$(call descend,$(@:_clean=),clean)
 
@@ -142,9 +148,12 @@ tmon_clean:
 freefall_clean:
 	$(call descend,laptop/freefall,clean)
 
+build_clean:
+	$(call descend,build,clean)
+
 clean: acpi_clean cgroup_clean cpupower_clean hv_clean firewire_clean lguest_clean \
 		perf_clean selftests_clean turbostat_clean usb_clean virtio_clean \
 		vm_clean net_clean iio_clean x86_energy_perf_policy_clean tmon_clean \
-		freefall_clean
+		freefall_clean build_clean libbpf_clean libsubcmd_clean liblockdep_clean
 
 .PHONY: FORCE