ARM: imx6q_logic: Cleanup boot sequence check

The board_boot_order() function currenly assumes that the boot source
is MMC/eMMC, but this isn't true for the NAND devices.

This patch cleans up board_boot_order() to check for NAND, SD, ESD,
MMC or EMMC.  Anything beyond these are not supported, so it will
default back to the serial downloader if any of those devices are not
available.

Fixes: 9fb50c68 ("ARM: imx6q_logic: Fix MMC2 booting")

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