CMSIS-NN: Update cmake to be able to use external toolchains
CMSIS-NN: Update cmake to be able to use external toolchains

ARM-software/CMSIS_5,JonatanAntoni/CMSIS_5,JonatanAntoni/CMSIS_5,JonatanAntoni/CMSIS_5,JonatanAntoni/CMSIS_5,ARM-software/CMSIS_5,ARM-software/CMSIS_5,ARM-software/CMSIS_5,JonatanAntoni/CMSIS_5,ARM-software/CMSIS_5,JonatanAntoni/CMSIS_5,ARM-software/CMSIS_5