Import the NetBSD mkubootimage tool for building uboot kernel images

[jhibbits]

Summary

The u-boot-tools mkimage does not build images bootable on powerpc, at
least when built from a powerpc system. The NetBSD mkubootimage tool builds
images bootable on powerpc, and buildable from powerpc. Import it as a tool for
now, but maybe make it an installed tool later

This is a straight port, with the only changes made to get it to build on
FreeBSD.

[jhibbits]

Remove .depend file

[manu]

Do we finally need it ?
Unless we need to create image for release images and using the u-boot-tools port isn't ok I dop't see a reason right now.

[jhibbits]

Upstream uboot and the u-boot-tools Port have been fixed, so this is no longer necessary.