uboot-imx

pxe: Fix crash if 'sysboot' is run without args

Previously, a NULL pointer dereference would occur if the 'sysboot'
command is executed without any arguments.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Simon Glass <sjg@chromium.org>