Enable the second sdcard slot found on some boards. Note that we do not
set CONFIG_MMC_SUNXI_SLOT_EXTRA for the SPL, as having it there is not useful,

Except for on the Mele-M3 where the second sdcard is an eMMC, from which the
device can also boot, and there we want to have both in the SPL, so that
a single u-boot binary can both from both. So for the M3 we do prefix the
defconfig setting with the special "+S:" syntax so that it applies to the
SPL too.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>