vexpress: iks: change to usb boot

Filesystem is preferred to be on usb for IKS build due to
slower through put over mmc.

Change-Id: I1150adc1c7a922da309c66d7d1c0e0460d19ae4f
Signed-off-by: Vishal Bhoj <vishal.bhoj@linaro.org>
2 files changed