imx: Introduce CONFIG_SPL_FORCE_MMC_BOOT to force MMC boot on falcon mode

This change tries to fix the following problem:

- The board boots (to be more precise - ROM loads SPL) from a slow SPI-NOR
  memory.
  As a result the spl_boot_device() will return SPI-NOR as a boot device
  (which is correct).

- The problem is that in 'falcon boot' the eMMC is used as a boot medium to
  load kernel from its partition.
  Calling spl_boot_device() will break things as it returns SPI-NOR device.

To fix this issue the new CONFIG_SPL_FORCE_MMC_BOOT Kconfig flag is
introduced to handle this special use case. By default it is not defined,
so there is no change in the legacy code flow.

Signed-off-by: Lukasz Majewski <lukma@denx.de>