* Adding common source location in ./src/platform
To enable the continued clean up of multiple files still in ./src/main/driver (more PRs to follow) that are specifically for AT32, APM32 and STM32
Source will be moved to MCU_COMMON_SRC where it is specifically for that MCU (or variant). The test will be to ensure no files in the MCU_EXCLUDES for SITL.
* Use of +=