wandboard: Set default environment to use zImage

wandboard: 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.

The zImage works for FSL Linux's kernel fork versions 3.0.35, 3.10.9,
and 3.10.17; this also works fine for mainline kernels.

Signed-off-by: John Weber <rjohnweber@gmail.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>