kbuild: Do not clean resolve_btfids if the output does not exist

Nathan reported issue with cleaning empty build directory:

  $ make -s O=build distclean
  ../../scripts/Makefile.include:4: *** \
  O=/ho...build/tools/bpf/resolve_btfids does not exist.  Stop.

The problem that tools scripts require existing output
directory, otherwise it fails.

Adding check around the resolve_btfids clean target to
ensure the output directory is in place.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/bpf/20210211124004.1144344-1-jolsa@kernel.org

Makefile

@@ -1088,8 +1088,14 @@ endif
 
 PHONY += resolve_btfids_clean
 
+resolve_btfids_O = $(abspath $(objtree))/tools/bpf/resolve_btfids
+
+# tools/bpf/resolve_btfids directory might not exist
+# in output directory, skip its clean in that case
 resolve_btfids_clean:
-	$(Q)$(MAKE) -sC $(srctree)/tools/bpf/resolve_btfids O=$(abspath $(objtree))/tools/bpf/resolve_btfids clean
+ifneq ($(wildcard $(resolve_btfids_O)),)
+	$(Q)$(MAKE) -sC $(srctree)/tools/bpf/resolve_btfids O=$(resolve_btfids_O) clean
+endif
 
 ifdef CONFIG_BPF
 ifdef CONFIG_DEBUG_INFO_BTF