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 a separate function before
the extend_recipe_sysroot prefunc else we'd wipe out the sysroot we just created.
This allows fetcher do_populate_sysroot dependencies to work correctly again.
I did try various other approaches and a seperate function with cleandirs was the
cleanest way to add this without code duplication or too much complexity.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
