[PATCH 1/2] selftests/lib.mk: fix INSTALL_RULE

This patch fixes the INSTALL_RULE to gracefully handle the case where
TEST_PROGS and TEST_PROGS_EXTENDED and TEST_FILES are not set. In this case,
install is called without any SOURCE arguments causing make install to fail.
The proposed fix is to loop over the items in these variables and only call
install if there is a test artifact present.

Signed-off-by: Tyler Baker <redacted>
---
 tools/testing/selftests/lib.mk | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/lib.mk b/tools/testing/selftests/lib.mk
index ee412ba..89dd785f 100644
--- a/tools/testing/selftests/lib.mk
+++ b/tools/testing/selftests/lib.mk

@@ -13,10 +13,12 @@ run_tests: all
 
 define INSTALL_RULE
 	mkdir -p $(INSTALL_PATH)
-	@for TEST_DIR in $(TEST_DIRS); do\
+	@for TEST_DIR in $(TEST_DIRS); do \
 		cp -r $$TEST_DIR $(INSTALL_PATH); \
 	done;
-	install -t $(INSTALL_PATH) $(TEST_PROGS) $(TEST_PROGS_EXTENDED) $(TEST_FILES)
+	@for ARTIFACT in $(TEST_PROGS) $(TEST_PROGS_EXTENDED) $(TEST_FILES); do \
+		install -t $(INSTALL_PATH) $$ARTIFACT; \
+	done;
 endef
 
 install: all