CMake: Call mbed_set_post_build API for setting post build operations

[rajkan01]

• Called mbed_set_post_build API for setting post-build operations
• Update CMake minimum version required from 3.18.2 to 3.19.0
• Replace mbed-os source path MBED_ROOT with MBED_PATH in CMakeLists.txt

[0xc0170]

Waiting for Circle CI to complete (soon will be)