configs: drop cubieboard_defconfig

This defconfig is affected by bug #8516, reported more than a year ago,
and nobody has updated the defconfig since then. In addition, it should
be migrated to the genimage mechanism instead of having its own
mkcubieboard.sh script.

Therefore, let's get rid of this defconfig.

Fixes bug #8516.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Carlo Caione <carlo.caione@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>