MA-14916-4 support dual bootloader for imx8m/imx8q

MA-14916-4 support dual bootloader for imx8m/imx8q

This commit enables dual bootloader feature for imx8m/imx8q, but
as commit 'a2018ab' already brings in some dual bootloader codes
when enabling fastboot support, so this commit won't be a complete
and standalone patch to introduce the dual bootloader feature.

This commit will do the following:
  1. clean up dual bootloader flow and add missing implementation.
  2. Merge the dual bootloader entry for fit and container to one
     function 'mmc_load_image_raw_sector_dual_uboot'.

Change-Id: Ic9410a48092cc05de599dd897fc912177e2a1fe1
Signed-off-by: Ji Luo <ji.luo@nxp.com>