xlat: extend syntax

* xlat/gen.sh: Implement #stop directive that stops preprocessor
at the line it has been encountered during the first pass.  This could
be used, for example, to provide fallback definitions for constants
without defining xlat tables.

xlat/gen.sh

@@ -135,6 +135,9 @@ gen_header()
 	while read line; do
 		LC_COLLATE=C
 		case $line in
+		'#stop')
+			exit 0
+			;;
 		'#conditional')
 			unconditional=
 			;;