rockchip: ringneck-px30/puma-rk3399: factor out storage medium selection

Ringneck PX30 and Puma RK3399 both have the same expectation with regard
to bootstd device order and U-Boot environment storage device, except
that Puma RK3399 also supports SPI Flash.

Let's move all of this into a common file where common logic can be put.

Cc: Quentin Schulz <foss+uboot@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/board/theobroma-systems/ringneck_px30/MAINTAINERS b/board/theobroma-systems/ringneck_px30/MAINTAINERS
index d764e26..06e1bea 100644
--- a/board/theobroma-systems/ringneck_px30/MAINTAINERS
+++ b/board/theobroma-systems/ringneck_px30/MAINTAINERS
@@ -3,6 +3,7 @@
 M:	Klaus Goger <klaus.goger@theobroma-systems.com>
 S:	Maintained
 F:	board/theobroma-systems/ringneck_px30
+F:	board/theobroma-systems/common
 F:	include/configs/ringneck_px30.h
 F:	arch/arm/dts/px30-ringneck*
 F:	configs/ringneck-px30_defconfig