selftests, bpftool: Skip the build test if not in tree

If selftests are copied over to another machine/location
for execution the build test of bpftool will obviously
not work, since the sources are not copied.
Skip it if we can't find bpftool's Makefile.
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20191119105010.19189-3-quentin.monnet@netronome.com

tools/testing/selftests/bpf/test_bpftool_build.sh

@@ -20,6 +20,10 @@ SCRIPT_REL_PATH=$(realpath --relative-to=$PWD $0)
 SCRIPT_REL_DIR=$(dirname $SCRIPT_REL_PATH)
 KDIR_ROOT_DIR=$(realpath $PWD/$SCRIPT_REL_DIR/../../../../)
 cd $KDIR_ROOT_DIR
+if [ ! -e tools/bpf/bpftool/Makefile ]; then
+	echo -e "skip:    bpftool files not found!\n"
+	exit 0
+fi
 
 ERROR=0
 TMPDIR=