- revert after #8689 is resolved

 - use make instead of ninja on OSX for now

 * use regular optimization levels for the code coverage build.
 * the mixer test fails intermittently when built without optimization

filtering files for code check seperately to enable fast use of git pre-commit hook to check code style
ask user to install pre-commit hook when code style is checked

- remove broken ccache setup

* move gcc 4.9 build to circleci

* travis-ci update to xcode 7.3

* travis-ci limit git fetching for OSX

* Makefile split firmware targets for CI

* OSX ccache