sstrip: make it a proper package

* Convert sstrip to a proper gentargets package
* Use openwrt svn version, it's basically the same one we used
* Change the hooks from old toolchain/sstrip to new package/sstrip
* Drop the old toolchain/sstrip directory
* sstrip for the target is now in Package -> Development

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>