Add option to turn off logging completely

Add new ETHOSU_LOG_ENABLE variable in the CMakeLists.txt to toggle
logging completely off, or else by the desired severity level.
Turning off the logging will also disable the LOG() macro.

Change-Id: I313a97fb1569000ae22637734ef4a60eedc56f70
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>