staging: Fix autoconf-native rebuild failure

When rebuilds are triggered, autoconf-native can fail with:

| DEBUG: Executing shell function update_gnu_config
| install: cannot stat '[BUILDPATH]tmp/work/x86_64-linux/autoconf-native/2.71-r0/recipe-sysroot-native/usr/share/gnu-config/config.guess': No such file or directory

which is due to update_gnu_config running before extend_recipe_sysroot.
This only happens rarely since usually the prepare_recipe_sysroot
function would already have set things up and only in the invalidated
task hash cases does this rebuild in this way from configure only.

Fix the code to prepend this function instead of appending which
resolves the ordering issue.

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