This concerns only debug builds. In BL2, we now pass a special value
to BL3-1 through x1. In BL3-1, we check that we receive the correct
value. This ensures that any platform set value is correctly passed
from BL2 to BL3-1.

Change-Id: Icb94d0e99ca445dcbe574a7d5e291659bba2209e