Update TFLM build configuration

- Renames build flags (TFLU_*->TFLM_*).
- Fix generator expressions in add_compile_definitions.
- Adds new build flag TFLM_OPTIMIZE_KERNELS_FOR. This is a new flag in
  TFLM, which is currently needed for CMSIS-NN Transpose conv.
  Since TFLM's own build files are not used it need to be set explicitly
  here.

Change-Id: I8ff01caabfc1c5d2fcc3fa36fcbd153525d93258
Signed-off-by: Måns Nilsson <mans.nilsson@arm.com>