Add new port sysutils/u-boot-tools

U-Boot tools for creating and analyzing u-boot images.
U-Boot images are used to embed kernels into a know format for u-boot.
It can also be FIT (Flattened Image Tree) images when the source describing
the image is written like a DTS (Device Tree Source).

Reviewed by: imp
Approved by: imp
Differential Revision: https://reviews.freebsd.org/D13455