u-boot: Add infrastructure to SPL verified boot

Add the necessary infrastructure to create a U-boot proper fitimage,
sign it (using the same keys as the kernel-fitimage), and put the public
key in the SPL binary so that verified SPL boot can be accomplished.

Signed-off-by: Klaus Heinrich Kiwi <klaus@linux.vnet.ibm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>