gluon-tls: add marker file

Allow checking for TLS support at runtime or for site checks.

package/gluon-tls/Makefile

@@ -6,7 +6,12 @@ include ../gluon.mk
 
 define Package/gluon-tls
   DEPENDS:=+libustream-mbedtls +ca-bundle
-  TITLE:=Dummy Package to select the current TLS implementation
+  TITLE:=Enable TLS support
+endef
+
+define Package/gluon-tls/install
+	$(INSTALL_DIR) $(1)/lib/gluon/features
+	touch $(1)/lib/gluon/features/tls
 endef
 
 $(eval $(call BuildPackageGluon,gluon-tls))