Add TFLM_TARGET parameter to TFLM build

* Adds TFLM_TARGET to tflite_micro.cmake
* This allows TFLM to be built without
  Arm Cortex-M libraries when building
  for other platforms

Change-Id: Ic4c80ae10bf42ae4bfcc11844f3e793921b7735f
Signed-off-by: Alex Tawse <alex.tawse@arm.com>