This commit fixes two bugs related to the usage of MODULES for CC2538-based platforms:
* newlib is no longer under `lib/newlib` but under `os/lib/newlib`.
* There is no such thing as `sys` under `arm/common`, so this module is removed.
Some platforms provide this header file because some examples used to assume its presence. Now that the examples no longer make this assumption, those stub headers are no longer required.
Previously, the CC2538 port would compile USB-related files from their original location under the CC2531 CPU dir. We are now removing the latter, but we need to keep those files.