MLECO-5415: Fix for CMSIS-NN usage

Fix for recent change to use CMake wrapper for building TensorFlow
Lite Micro - CMSIS-NN kernels were not being included in the build
heavily impacting performance for CPU-only path. This patch has a
minor change to correctly set `CORE_SOFTWARE_ACCELERATOR` CMake
variable to ensure the CMake wrapper actually adds CMSIS-NN to the
build.

Also, with the CMake build flow for CMSIS-NN and TensorFlow Lite
Micro together, the linker scripts have been modified to allow object
file name based memory placement to still work as it was for Makefile
based flow earlier.

Change-Id: Iedd91d4db7bbaa40603df6a7e93e99274be547d6
Signed-off-by: Kshitij Sisodia <kshitij.sisodia@arm.com>