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>
sunxi: When we've both mmc0 and mmc2, detect from which one we're booting

sunxi SOCs can boot from both mmc0 and mmc2, detect from which one we're
booting, and make that one "mmc dev 0" so that a single u-boot binary can
be used for both the onboard eMMC and for external sdcards.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
M3_wip