uboot-phytec

MLK-14250-1 mx6slevk: Add plugin boot support

Add plugin.S for plugin boot initialization. Need to set "CONFIG_USE_IMXIMG_PLUGIN"
for this feature.
Add the build configuration "mx6slevk_plugin_defconfig" to use the plugin.

Signed-off-by: Ye Li <ye.li@nxp.com>