buildroot

via_imx6_vab820_defconfig: remove broken defconfig

This defconfig has been broken due to gcc5 for more than half a year,
and nobody stepped up to fix it. Remove it, as well as it board
directory.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Gergely Imreh <imrehg@gmail.com>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>