Add VC4 drivers in Mesa CI
Adds testing for OpenGL ES 2.0 for VC4 drivers, used in the Raspberry Pi 3.
It uses real Raspberry devices, that are booted using NFS/TFTP with the testsuite and the Mesa drivers compiled for armhf architecture. The devices are powered on/off through a Power-over-Ethernet switch, and monitor through a serial to know when the testing finishes.