Why a Datum Fork?

After updating to STLinux release 4.1.0, optee is required by the boot chain. The STMicro fork of optee-os is stuck at v3.16.0, but nerves_sytem_br and buildroot march onward into incompatibilities with optee-os v3.16.0 crop up and you cannot build.

This fork allows us to revert the optee packages in buildroot back to v3.15.0, allowing the system to be built. I picked v3.15.0 instead of v3.16.0 becuase that is what STMicro uses in their ecosystem release.

There may be other, better ways of doing this, but I don't know them. The fork allows us to patch buildroot during the nerves_system_brick build process. I considered added custom packages with the required versions, but I read somewhere that you cannot duplicate package names, and having different package names for the optee stuff might wreck other aspects of the build process.