x86: zimage: Update cmdline parameter to be an env var

With the updated changes to bootargs substitution[1], the zboot command
needs to be updated to get its command line from an environment variable
instead of a memory address. This is because the command-line string must
be updated to convert %U to ${uuid}, etc.

In any case it is more flexible to use a environment variable and it is
best to do this before the release to avoid a subsequent change.

Update the command accordingly.

[1] http://patchwork.ozlabs.org/project/uboot/list/?series=212481



Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>