Do not convert any of the 16bit index variables at this time: the
enforced limit of TXTNODE_MAX should ensure that they are sufficient,
and there are thousands of these variables all around sw.
(also, STRING_LEN/STRING_NOTFOUND is used as a "not found" value all
over the place)

Change-Id: Id26a145701de731470b0bcb95359ed62d4d6d579