Promote MBED_TOOLCHAIN_FILE_USED to cache variable so
its previous state can be recalled when re-building. This
is because CMAKE_TOOLCHAIN_FILE is also a cache variable and its
value is set on first run when it has not been set.
Rename the CMake variable expected from applications from MBED_ROOT to
MBED_PATH to be consistent with the naming used for the configuration file
(MBED_CONFIG_PATH) also expected from applications.
These settings like enable_language should be done in the application and just once.
We hit the issue when you expose sources to an app (interface or public), CMake errors as some of the internal settings have not been configured.
Jamie Smith
Hugues Kamba
Ladislas de Toldi
Martin Kojtal