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.
Directories that start with special prefixes (TARGET_, FEATURE_, COMPONENT_) are added to the build based on Mbed target configuration from targets.json instead of calling utility function mbed_add_cmake_directory_if_labels().