oeqa/utils: targetbuild: take proxy into account

A previous commit broke downloads
when proxies are involved, let's fix it.

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
testimage: fail if no package manifest is found

Sometimes we may forget to actually build the image
we want to test (when testimage task is called manually).
Instead of an ugly traceback we should fail nicely.
The manifest is written after the rootfs so this ensures
the image was actually built.

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>