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

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

this commit is a merge of three patches from imx_v2018.03 as below:

1.
commit dbcf1e3cc079d2f1b3df6c4c9ec3a34d0c05eb4c
Author: Luo Ji <ji.luo@nxp.com>
Date:   Fri Jun 8 10:31:11 2018 +0800

    [iot] Support dual bootloader in SPL

    Move the A/B slot check to SPL, the A/B slot switch
    workflow is just like what we have in libavb_ab.

    Test: A/B select works fine on imx8m.

2.
commit 71562aae3b8123ccd7503e596e478951568fcd24
Author: Ji Luo <ji.luo@nxp.com>
Date:   Mon Jan 14 18:28:08 2019 +0800

    MA-13938 [Android] imx8q: Support dual bootloader feature

    Support dual bootloader feature for imx8q which uses the
    container format. Move the A/B slot select and verify to
    SPL stage, the bootloader rollback index will be stored
    at the last 8K bytes of eMMC rpmb storage.

    Test: Boot and rbindex verify pass on imx8q.

Change-Id: Ic9410a48092cc05de599dd897fc912177e2a1fe1
Signed-off-by: faqiang.zhu <faqiang.zhu@nxp.com>