u-boot: basic support of dtb append for verified boot
This introduces a new uboot-sign.class to support U-Boot verified boot.
This part delivers the new class file, with related environment variables, and
a new task intended to run before do_install task and which performs the
concatenation of the u-boot-nodtb.bin and the device tree blob. The 'cat'
command used overrides the u-boot.bin in both DEPLOYDIR & build dir to
propagate the changes in later tasks (do_install, do_package, etc.)
Signed-off-by: Yannick Gicquel <yannick.gicquel@iot.bzh> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
