u-boot: Move definitions to common locations

Move some definitions from u-boot.inc into uboot-config.bbclass and
similarly from kernel-fitimage.bbclass into uboot-sign.bbclass, so that
they can be useful when signing the U-boot proper fitimage, for a
verified-boot SPL.

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