trusty: Mark vexpress-a15 target as 32-bit We rely on KERNEL_32BIT being set for Trusty targets that are 32-bit platforms. Adds this make variable definition to the vexpress target. Test: build.py vexpress-a15-trusty Bug: None Change-Id: I3cdf96efd7b10b0d56b212542bc1bc9632a1fb0f