CMake: Remove requirement for apps to call our custom functions (or move the custom functions out of mbed-os/CMakeLists)

[rwalton-arm]

Description of defect

Ideally we wouldn't place a hard requirement on the consumer to call custom CMake functions to build successfully. If we must provide custom CMake functions they should be made available through include statements at the top-level and not as a side-effect of add_subdirectory (currently some of our exported functions are made available through adding the mbed-os subdirectory).

• Remove requirement for app to call custom functions (if possible)
• If that's not possible (e.g to run objcopy) move the API functions to .cmake modules so they're self contained and dependencies are more easily visible

Target(s) affected by this defect ?

All

Toolchain(s) (name and version) displaying this defect ?

All

What version of Mbed-os are you using (tag or sha) ?

master

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

All

How is this defect reproduced ?

Remove mbed custom function from an example application's CMakeLists.txt

[ciarmcom]

Thank you for raising this detailed GitHub issue. I am now notifying our internal issue triagers.
Internal Jira reference: https://jira.arm.com/browse/IOTOSM-3781