minnowboard_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 the files it
references. Update the readme to remove references to MinnowBoard.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
4 files changed