Add SLAC-specific cross targets to CI

[JJL772]

As part of the S3DF migration, we're trying to rebuild our cross toolchains in a reproducible manner to ease the current, and any future, transitions to a new Linux distribution. Just so happens the best way to do clean-room rebuilds is in a Docker container :)

SLAC's buildroot 2019.08 toolchain (and soon 2016.11.1) is available here: https://github.com/JJL772/slac-buildroot-docker

RTEMS is next on the list.

With these containers, we should be able to get CI (and automated tests) working for SLAC-specific targets such as RTEMS 4.10.2 and buildroot 2019.08/2016.11.1.

• buildroot-2019.08 (x86_64, i686, arm_zynq)
• buildroot-2016.11.1 (x86_64, i686, arm_zynq)
• RTEMS 4.10.2 (PowerPC, m68k, i386)