default-distrovars.inc: Switch connectivity check to a yoctoproject.org page

example.com is proving unreliable at present so switch to our own connectivity
page instead. That page is very simple avoiding app overhead on our web server
which was an original reason for switching to example.com.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit dc6b043c)
Signed-off-by: Steve Sakoman <steve@sakoman.com>