CMake: Add requirements.txt file for required Python modules

The memmap script depends on external Python modules.
The added file captures the required packages to
be installed.

tools/cmake/README.md

@@ -49,6 +49,7 @@ Supported examples can be identified by the presence of a top level `CMakeLists.
 Prerequisities:
 - CMake >=3.19.0
 - mbed-tools >=4.0.0
+- Python modules defined in [`tools/cmake/requirements.txt`](./requirements.txt)
 
 From the application root or wherever `mbed-os.lib` is found, run the following command to:
  * create the Mbed OS configuration CMake module

tools/cmake/requirements.txt

@@ -0,0 +1,3 @@
+prettytable==0.7.2
+future==0.16.0
+Jinja2>=2.10.1,<2.11