Append androidboot.soc_type based on the imx soc type,
only add imx6/7 support.

Change-Id: I3ae18bff42b434eb77728a7db70dd3baf6d7e0a6
Signed-off-by: guoyin.chen <guoyin.chen@nxp.com>
Signed-off-by: Richard Liu <xuegang.liu@nxp.com>