sepolicy: Start CM Common sepolicy

Rather than having to maintain out of tree changes, it is often
easier to maintain a hiearchy of changes, starting with the vendors
common config file. From there, inheriting products can pick up a base
and start to add or remove certain bits from it, making use of the
BOARD_SEPOLICY_* functions documented in external/sepolicy/README.

Change-Id: I28a4aaf6c126535f0a88001582641b234a750015