Update: hellocv native + xc

[HesselM]

Update cmake-files and toolchain such that the hellocv example can both run native (x86/x64) in VM and on the RPi (arm)

• Install OpenCV 3.2.0 in VM
• Update hello/ocv/CMakeLists.txt to compile natively
• Update rpi-generic-toolchain.cmake to compile example via `-D CMAKE_TOOLCHAIN_FILE'
• Update guide

[HesselM]

fixed