tflite_micro: Disable more warnings

As TensorFlow Lite Micro is a third party library, declare include
directories as SYSTEM directories, to avoid warnings from applications
and libs that use the header files.

Add more compiler options to disable warnings when building the target.

Change-Id: I15054d064fecd870de09629df7211ebe1359cd7a
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>