Adds a new `cas` directory to the ports/linux project. This contains
an end-to-end demo of OEMCrypto CAS functionality, using the Linux
tee_simulator as a base.
Test: from ports/linux/cas dir: `CDM_DIR=~/work/cdm-dupe ./scripts/build.sh && CDM_DIR=~/work/cdm-dupe ./scripts/run.sh`
Merged from https://widevine-internal-review.googlesource.com/178250
Change-Id: I781b403100ad2e069d99650d9ddae8e7acbc309a
0dbc42f10e