ARM: imx6q_logic: Fix MMC2 booting

With the reverting of a previous change to spl_boot_device,
this board needs a new solution to determining if we're booting
from MMC1 or MMC2.

This patch creates board_boot_order function which overrides the
standard, and returns not only MMC1, or MMC2, but also can fall
back to NAND or the serial downloader should other boot options
fail.

Signed-off-by: Adam Ford <aford173@gmail.com>