libbpf: Clean up bpf_helper_defs.h generation output

bpf_helpers_doc.py script, used to generate bpf_helper_defs.h, unconditionally
emits one informational message to stderr. Remove it and preserve stderr to
contain only relevant errors. Also make sure script invocations command is
muted by default in libbpf's Makefile.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200113073143.1779940-3-andriin@fb.com

scripts/bpf_helpers_doc.py

@@ -158,8 +158,6 @@ class HeaderParser(object):
                 break
 
         self.reader.close()
-        print('Parsed description of %d helper function(s)' % len(self.helpers),
-              file=sys.stderr)
 
 ###############################################################################
 

tools/lib/bpf/Makefile

@@ -183,7 +183,7 @@ $(BPF_IN_STATIC): force elfdep zdep bpfdep $(BPF_HELPER_DEFS)
 	$(Q)$(MAKE) $(build)=libbpf OUTPUT=$(STATIC_OBJDIR)
 
 $(BPF_HELPER_DEFS): $(srctree)/tools/include/uapi/linux/bpf.h
-	$(Q)$(srctree)/scripts/bpf_helpers_doc.py --header 		\
+	$(QUIET_GEN)$(srctree)/scripts/bpf_helpers_doc.py --header \
 		--file $(srctree)/tools/include/uapi/linux/bpf.h > $(BPF_HELPER_DEFS)
 
 $(OUTPUT)libbpf.so: $(OUTPUT)libbpf.so.$(LIBBPF_VERSION)