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>
diff --git a/board/amlogic/odroid-c2/MAINTAINERS b/board/amlogic/odroid-c2/MAINTAINERS
index 699850f..fd74d53 100644
--- a/board/amlogic/odroid-c2/MAINTAINERS
+++ b/board/amlogic/odroid-c2/MAINTAINERS
@@ -1,5 +1,6 @@
 ODROID-C2
 M:	Beniamino Galvani <b.galvani@gmail.com>
+M:	Neil Armstrong <narmstrong@baylibre.com>
 S:	Maintained
 F:	board/amlogic/odroid-c2/
 F:	include/configs/odroid-c2.h