website: Add a Google+ presence

Since we use Google+ to share our news, this add a logo on the menu bar,
which links to the Buildroot profile.

[Peter: change link to Buildroot account]
Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>