apps: Add Keyword Detection ML application

* Add all source and header files to build the application
* Update open-iot-sdk-toolchain version to include the `-fno-rtti` C++
  flag
* Remove no longer needed `--specs=nosys.specs` flag as include
   already in the updated version of open-iot-sdk-toolchain
* Update build and run scripts to support keyword-detection application
* Add documentation for the Keyword Detection application

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>
Signed-off-by: Ahmed Ismail <Ahmed.Ismail@arm.com>