tc2: add u-boot build support but keep it disabled

Add support for building U-Boot for TC2, however, keep it disabled for
now because there are problems either in U-Boot of the Linux kernel that
mean the board doesn't boot.

Change-Id: I6130ba2bcac697fee430a3512cace057b3759f9b
Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>