selftests/bpf: Copy extras in out-of-srctree builds

Building selftests in a separate directory like this:

    make O="$BUILD" -C tools/testing/selftests/bpf

and then running:

    cd "$BUILD" && ./test_progs -t btf

causes all the non-flavored btf_dump_test_case_*.c tests to fail,
because these files are not copied to where test_progs expects to find
them.

Fix by not skipping EXT-COPY when the original $(OUTPUT) is not empty
(lib.mk sets it to $(shell pwd) in that case) and using rsync instead
of cp: cp fails because e.g. urandom_read is being copied into itself,
and rsync simply skips such cases. rsync is already used by kselftests
and therefore is not a new dependency.
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210224111445.102342-1-iii@linux.ibm.com

tools/testing/selftests/bpf/Makefile

@@ -382,11 +382,12 @@ $(TRUNNER_EXTRA_OBJS): $(TRUNNER_OUTPUT)/%.o:				\
 	$$(call msg,EXT-OBJ,$(TRUNNER_BINARY),$$@)
 	$(Q)$$(CC) $$(CFLAGS) -c $$< $$(LDLIBS) -o $$@
 
-# only copy extra resources if in flavored build
+# non-flavored in-srctree builds receive special treatment, in particular, we
+# do not need to copy extra resources (see e.g. test_btf_dump_case())
 $(TRUNNER_BINARY)-extras: $(TRUNNER_EXTRA_FILES) | $(TRUNNER_OUTPUT)
-ifneq ($2,)
+ifneq ($2:$(OUTPUT),:$(shell pwd))
 	$$(call msg,EXT-COPY,$(TRUNNER_BINARY),$(TRUNNER_EXTRA_FILES))
-	$(Q)cp -a $$^ $(TRUNNER_OUTPUT)/
+	$(Q)rsync -aq $$^ $(TRUNNER_OUTPUT)/
 endif
 
 $(OUTPUT)/$(TRUNNER_BINARY): $(TRUNNER_TEST_OBJS)			\