Build: Move out some codes from set_config.cmake

CMake config and Kconfig system will be separated later.
Moving out some common codes from set_config.cmake

- Add pre_config.cmake for common codes
  - Move out platform preload.cmake to pre_config.cmake
  - Move out Library Model deprecation message to pre_config.cmake
  - Move out CMAKE_BUILD_TYPE setup to pre_config.cmake
  - Error checks (Library Model, TFM_SYSTEM_DSP and CMAKE_GENERATOR) are
    gathered together into pre_config.cmake
- Add post_config.cmake for common codes
  - Move out the tfm_config setup from set_config.cmake to
    post_config.cmake.
  - Move out tfm_build_log_config.cmake inclusion to post_config.cmake
  - Move test repo fetch and config are separated and move to
    post_config.cmake

Change-Id: Idf39766d4f7c3e3933443c7eef31b9e0479064ea
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
6 files changed