Add --exclude option (to make uClibc-0.9.28 headers install using busybox tar).

Add --exclude option (to make uClibc-0.9.28 headers install using busybox tar).

I have no idea how to apply bb_getopt_complementally to a --longopt that
has no short option.  The documentation from vodz has a bad case of
babelfish poisoning, and I can't understand it.  It sort of seems to
suggest there is a way, but what it is I have no idea.  So I used \n as
the short option, which is fairly unlikely to be used for something else. :)