board: amlogic: factorise gxbb boards

The nanopi-k2 and the odroid-c2 are similar enough to be supported
by the same u-boot board. This change use odroid-c2 u-boot board
for the nanopi-k2 as well. Dedicated defconfig are kept to customize
the names and device tree.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>