Add continuous integration

• CI preparations
  • Prepare FreeRTOS tests (Demo/RISC5_LCC/main.c) to be run in CI
    • Move to Demo/RISC5_LCC/tests to be on the same level as the demo apps blinky and full
    • Activate all (working) tests in FreeRTOSConfig.h
    • Remove IntQueueTimer.{c,h}, since we won't support interrupt nesting for the time being
    • Add a CI_MODE build flag and modifiy the tests to return a status code for automatic testing in CI when the flag is set
  • Prepare all tests and demos for compilation in CI
    • Add a Makefile to build all tests under in driver_tests at once
    • Cleanup
      • Include common.mk in all Makefiles
        • under FreeRTOS
        • under FreeRTOS-Plus
      • Refactor .gitignore
        • remove modify top level .gitignore
        • Add nested .gitignore to blinky and full
        • Add nested .gitignore to driver_tests
        • Add nested .gitignore to projects under FreeRTOS-Plus
• Create .gitlab-ci.yml
  • figure out how we can use shared runners
    • tagging jobs with alpine seems to be the only way to get a linux image we can use to install the required build tools
  • build stage
    • clone and compile toolchain, export simulator binary as artifact for test stage
    • build projects under FreeRTOS and export build-artifacts
    • build projects under FreeRTOS-Plus and export build-artifacts
  • test stage
    • run Demo/RISC5_LCC/tests in simulator