stop the build if LIB_TAG is set at all; some users expect it to work while it doesn't

lib/functions/configuration/main-config.sh

@@ -347,6 +347,8 @@ function do_extra_configuration() {
 		This runs *after* user_config. Don't change anything not coming from other variables or meant to be configured by the user.
 	EXTENSION_PREPARE_CONFIG
 
+	error_if_lib_tag_set # make sure users are not thrown off by using old parameter which does nothing anymore
+
 	# apt-cacher-ng mirror configurarion
 	APT_MIRROR=$DEBIAN_MIRROR
 	if [[ $DISTRIBUTION == Ubuntu ]]; then

lib/functions/general/deprecations.sh

@@ -5,3 +5,10 @@ function error_if_kernel_only_set() {
 		return 1
 	fi
 }
+
+function error_if_lib_tag_set() {
+	if [[ "x${LIB_TAG}x" != "xx" ]]; then
+		exit_with_error "LIB_TAG is set.This is not supported anymore. Please remove it, and manage the git branches manually."
+		return 1
+	fi
+}