wandboard-dual: Make linux-wandboard preferred provider

With the addition of the linux-wandboard recipes, which build the
Wandboard kernel from the 'official' kernel source for Wandboard, change
the PREFERRED_PROVIDER to 'linux-wandboard'.
linux-imx (3.0.35): Remove wandboard from recipe

With the addition of the linux-wandboard recipe, we remove wandboard
support from the linux-imx recipes in order to prevent confusion and keep
all Wandboard effort focused on the 'official' Wandboard kernel.
images: Add video test image

video-image-test is an image adapted from fsl-image-test to incorporate
additional gstreamer plugins.