native: Use deltask to remove tasks which have no purpose

core-image-sato has 47703 inter task dependencies before this patch
and 29883 afterwards which is a significant worthwhile task graph
simplification.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
externalsrc: Use deltask API

Now we have deltask API, stop poking around bitbake internal variables.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
ptest: Fix dependency/ordering issue

We've seen occasional issues on the autobuilder where files appear during
do_populate_sysroot and this is due to the ptest installation happening
in parallel. This fixes the order to be deterministic.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
ptest: Disable for native recipes

We don't use or otherwise care about ptest for native recipes. Its therefore
pointless to take the performance hit for them and we can disable them.

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