tools: add fdt_add_pubkey

Having to use the -K option to mkimage to populate U-Boot's .dtb with the
public key while signing the kernel FIT image is often a little
awkward. In particular, when using a meta-build system such as
bitbake/Yocto, having the tasks of the kernel and U-Boot recipes
intertwined, modifying deployed artifacts and rebuilding U-Boot with
an updated .dtb is quite cumbersome. Also, in some scenarios one may
wish to build U-Boot complete with the public key(s) embedded in the
.dtb without the corresponding private keys being present on the same
build host.

So this adds a simple tool that allows one to disentangle the kernel
and U-Boot builds, by simply copy-pasting just enough of the mkimage
code to allow one to add a public key to a .dtb. When using mkimage,
some of the information is taken from the .its used to build the
kernel (algorithm and key name), so that of course needs to be
supplied on the command line.

Signed-off-by: Roman Kopytin <Roman.Kopytin@kaspersky.com>
Signed-off-by: Ivan Mikhaylov <fr0st61te@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Cc: Rasmus Villemoes <rasmus.villemoes@prevas.dk>