Fix make script to report build failure

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>
diff --git a/Makefile b/Makefile
index 31019fa..bc92298 100644
--- a/Makefile
+++ b/Makefile
@@ -14,12 +14,12 @@
 
 examples:
 	@for example in $(EXAMPLE_LIST); do \
-		$(MAKE) -C $$example CROSS_COMPILE="$(HOST_CROSS_COMPILE)"; \
+		$(MAKE) -C $$example CROSS_COMPILE="$(HOST_CROSS_COMPILE)" || exit -1; \
 	done
 
 examples-clean:
 	@for example in $(EXAMPLE_LIST); do \
-		$(MAKE) -C $$example clean; \
+		$(MAKE) -C $$example clean || exit -1; \
 	done
 
 prepare-for-rootfs: examples