This commit creates a `common` directory, aimed to host drivers supported by multiple boards of the CC13xx/CC26xx family.
We move the Sensortag SPI and External Flash drivers to this location and we change the Sensortag build system to pull the respective files from therein.
This commit updates the srf06-cc26xx platform by adding support for the CC1310EM.
We generalise the way this platform selects CPU/board so that we can easily add more combinations in the future. These changes have implication on how to build for different devices, so make sure to have a look at the updated README