staging: Fix sysroot problem with populate_sysroot dependencies on do_fetch

staging: Fix sysroot problem with populate_sysroot dependencies on do_fetch

Dependencies on svn:// urls were failing as the cleandirs on do_fetch was destroying
any sysroot setup by the extend_recipe_sysroot function.

Add code so that if the task do_fetch, we move the cleandirs to the extend_recipe_sysroot
task else we'd wipe out the sysroot we just created.

This allows fetcher do_populate_sysroot dependencies to work correctly again.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>