MLK-12434-1: imx: dynamic setting mmcdev and mmcroot

MLK-12434-1: imx: dynamic setting mmcdev and mmcroot

Align to imx_v2015.04, dynamic setting mmcdev and mmcroot.
Then when boot linux, we can have correct "root=/dev/mmcblk[x]p2"

Signed-off-by: Peng Fan <peng.fan@nxp.com>
(cherry picked from commit b46b99a901eb194e81fc4836ee2259ad8857f4d3)
(cherry picked from commit 6f6a828fbe7478efd5932c302e6368877107bbca)
(cherry picked from commit bb628be4e993e98fb2fe8fc6af7b16e706d0f32d)
(cherry picked from commit 8e56914b4f536094bdf2301e347be02e0ccfee42)
(cherry picked from commit 3352f12289b3979a329486620711ecc2c2fb9fbd)