uboot-imx

mx6slevk: Set default environment to use zImage

Change the default environment to use zImage instead of uImage, this
requires changes to the default environment to load a file named
zImage instead of uImage, and to use the 'bootz' command instead of
'bootm' when booting the kernel.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>