perf tools: Do not display extra info when there is nothing to build

Even with fully built tree, we still display extra output when make is
invoked, like:

  $ make
    BUILD:   Doing 'make -j8' parallel build
    DESCEND  plugins
  make[3]: Nothing to be done for 'plugins/libtraceevent-dynamic-list'.

Changing the make descend directly to plugins directory, which quiets
those messages down.
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Khuong <pvk@pvk.ca>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200507095024.2789147-2-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Makefile.perf

@@ -278,6 +278,7 @@ strip-libs = $(filter-out -l%,$(1))
 
 ifneq ($(OUTPUT),)
   TE_PATH=$(OUTPUT)
+  PLUGINS_PATH=$(OUTPUT)
   BPF_PATH=$(OUTPUT)
   SUBCMD_PATH=$(OUTPUT)
   LIBPERF_PATH=$(OUTPUT)
@@ -288,6 +289,7 @@ else
 endif
 else
   TE_PATH=$(TRACE_EVENT_DIR)
+  PLUGINS_PATH=$(TRACE_EVENT_DIR)plugins/
   API_PATH=$(LIB_DIR)
   BPF_PATH=$(BPF_DIR)
   SUBCMD_PATH=$(SUBCMD_DIR)
@@ -297,7 +299,7 @@ endif
 LIBTRACEEVENT = $(TE_PATH)libtraceevent.a
 export LIBTRACEEVENT
 
-LIBTRACEEVENT_DYNAMIC_LIST = $(TE_PATH)plugins/libtraceevent-dynamic-list
+LIBTRACEEVENT_DYNAMIC_LIST = $(PLUGINS_PATH)libtraceevent-dynamic-list
 
 #
 # The static build has no dynsym table, so this does not work for
@@ -756,10 +758,10 @@ $(LIBTRACEEVENT): FORCE
 	$(Q)$(MAKE) -C $(TRACE_EVENT_DIR) $(LIBTRACEEVENT_FLAGS) O=$(OUTPUT) $(OUTPUT)libtraceevent.a
 
 libtraceevent_plugins: FORCE
-	$(Q)$(MAKE) -C $(TRACE_EVENT_DIR) $(LIBTRACEEVENT_FLAGS) O=$(OUTPUT) plugins
+	$(Q)$(MAKE) -C $(TRACE_EVENT_DIR)plugins $(LIBTRACEEVENT_FLAGS) O=$(OUTPUT) plugins
 
 $(LIBTRACEEVENT_DYNAMIC_LIST): libtraceevent_plugins
-	$(Q)$(MAKE) -C $(TRACE_EVENT_DIR) $(LIBTRACEEVENT_FLAGS) O=$(OUTPUT) $(OUTPUT)plugins/libtraceevent-dynamic-list
+	$(Q)$(MAKE) -C $(TRACE_EVENT_DIR)plugins $(LIBTRACEEVENT_FLAGS) O=$(OUTPUT) $(OUTPUT)libtraceevent-dynamic-list
 
 $(LIBTRACEEVENT)-clean:
 	$(call QUIET_CLEAN, libtraceevent)