build: Add __ZEPHYR__ to Makefile

This commit adds the __ZEPHYR__ define to the main Makefile.
This new define may help to drive conditional compilation when
writing multi-platform applications.

Change-Id: I81a37f9c86fa7f85bbac7cd0c0cd4150cbff1911
Signed-off-by: Flavio Santes <flavio.santes@intel.com>