mx6cuboxi: customize board_boot_order to access eMMC

In SPL legacy code only one MMC device is created, based on BOOT_CFG
register, which can be either SD or eMMC. In this context
board_boot_order return always MMC1 when configure to boot from
SD/eMMC. After switching to DM both SD and eMMC devices are created
based on the information available on DT, but as board_boot_order
only returns MMC1 is not possible to boot from eMMC.

This patch customizes board_boot_order taking into account BOOT_CFG
register to point to correct MMC1 / MMC2 device. Additionally, handle
IO mux for the desired boot device.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>