message: mstarch: updating RPI toolchain to be environment driven, if specified
diff: set(CMAKE_SYSTEM_NAME "arm-linux-gnueabihf")
  # Location of pi toolchain
  -set(PI_TOOLCHAIN "/opt/rpi/tools/arm-bcm2708/arm-rpi-4.9.3-linux-gnueabihf/")
  -
  +set(RPI_TOOLCHAIN "$ENV{RPI_TOOLCHAIN_DIR}")
  +if ("${RPI_TOOLCHAIN}" STREQUAL "")
  + set(RPI_TOOLCHAIN "/opt/rpi/tools/arm-bcm2708/arm-rpi-4.9.3-linux-gnueabihf/")
  +endif()
  +# Check toolchain directory exists
  +IF(NOT EXISTS "${RPI_TOOLCHAIN}")
  + message(FATAL_ERROR "RPI toolchain not found at ${RPI_TOOLCHAIN}. Install it, set RPI_TOOLCHAIN_DIR environment variable, or choose another toolchain")
  +endif()
  +message(STATUS "Using RPI toolchain at: ${RPI_TOOLCHAIN}")
  # specify the cross compiler
  -set(CMAKE_C_COMPILER "${PI_TOOLCHAIN}/bin/arm-linux-gnueabihf-gcc")
  -set(CMAKE_CXX_COMPILER "${PI_TOOLCHAIN}/bin/arm-linux-gnueabihf-g++")
  +set(CMAKE_C_COMPILER "${RPI_TOOLCHAIN}/bin/arm-linux-gnueabihf-gcc")
  +set(CMAKE_CXX_COMPILER "${RPI_TOOLCHAIN}/bin/arm-linux-gnueabihf-g++")
  # where is the target environment
  -set(CMAKE_FIND_ROOT_PATH "${PI_TOOLCHAIN}")
  +set(CMAKE_FIND_ROOT_PATH "${RPI_TOOLCHAIN}")
  # search for programs in the build host directories
  set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
  