configure: Don't emit _PRESENT macros

Do not put macros like MBED_CONF_FILESYSTEM_PRESENT or
MBED_CONF_EVENTS_PRESENT into the applicaiton's generated
mbed_config.cmake file. We will depend on Mbed OS CMake files for each
of these components to define the _PRESENT macro in a
target_compile_definitions() block for the e.g. filesystem or events
CMake targets.

news/20200929160957.misc

@@ -0,0 +1 @@
+Don't emit _PRESENT macros. Depend on Mbed OS to define these from CMake.

src/mbed_tools/build/_internal/config/config.py

@@ -104,6 +104,8 @@ def from_sources(cls, sources: Iterable[Source]) -> "Config":
         config = Config()
         for source in sources:
             for key, value in source.config.items():
+                if key.endswith(".present"):
+                    continue
                 _create_config_option(config, key, value, source)
             for key, value in source.overrides.items():
                 if key in IGNORED_OVERRIDE_KEYS_IN_SOURCE: