sunxi: Enable a second mmc socket as boot target in the environment

Some sunxi-based boards (such as the Olimex A20-SOM-EVB) have a
second MMC socket. This socket is not bootable hardware-wise,
i.e. u-boot itself cannot be loaded from it, but once u-boot has
started, the second socket can be used in the boot process
provided by config_distro_bootcmd.h.

If a second MMC socket is present, place it in the boot order
after the first MMC socket.

Signed-off-by: Karsten Merker <merker@debian.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>