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>