Don't fail build of otatools if there are no device certificates

Bug: 23552169
Change-Id: I32723701d952ef2243ce0234132fa9876f0f21b6
(cherry picked from commit 8d771971212069f77851163f437c782f581bf6db)
diff --git a/core/Makefile b/core/Makefile
index 7c52f4e..c35ca6e 100644
--- a/core/Makefile
+++ b/core/Makefile
@@ -1302,7 +1302,7 @@
 	$(hide) rm -rf $@ $(zip_root)/releasetools/*.pyc
 	$(hide) (cd $(zip_root) && zip -qry $(abspath $@) bin framework releasetools)
 	$(hide) zip -qry $(abspath $@) build/target/product/security/
-	$(hide) find device vendor -name \*.pk8 -o -name \*.x509.pem | xargs zip -qry $(abspath $@)
+	$(hide) find device vendor -name \*.pk8 -o -name \*.x509.pem | xargs zip -qry $(abspath $@)>/dev/null || true
 
 .PHONY: otatools-package
 otatools-package: $(BUILT_OTATOOLS_PACKAGE)