toolchain: do staging symlink

Make an output/staging -> sysroot symlink just for compatibility
purposes.
This may be gone in the future so don't count on it, use $(STAGING_DIR)
instead.

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