uboot-imx

sunxi: Turn MMC_SUNXI_SLOT_EXTRA into a proper Kconfig option

Note we also drop the SPL check for initializing the 2nd mmc slot, the SPL
check is not necessary with Kconfig, because only options explicitly marked
as also being for the SPL get set during SPL builds.

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