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)
Peng Fan