buildroot

configs/imx6q-sabresd: Drop the 'quad' notation

Now that SPL is supported a single image can run on the different
variants of imx6 sabresd boards: imx6q, imx6dl and imx6qp.

So drop the 'quad' prefix from defconfig, directory and readme file
to make it generic.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>