Fix build issue

Product group in their wisdom are using capabilities from cmake 3.15. This is not available on Ubuntu 18.04 (and probably many other systems). Modified the build to remove the offending line from the cmake file to enable builds with earlier versions of cmake.

Actually pull in latest C SDK this time

Apparently I screwed up the last release and picked up the wrong version of the C SDK. Fix that.

Mostly Housekeeping

Clean up some code and improve documentation.

Pulls the Azure SDK for C from a commit rather than a release in order to include a fix.

Initial stable drop

Still a work in progress but is stable enough for use by https://github.com/markrad/azure-embedded-sdk-rs.