config: Fix target.requires

Apparently the old tools allowed a user to specify a required lib in the
"target_overrides" section of the config, even if no root "requires" are
defined. This commit adds support for this workflow.

Fixes #162

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

@@ -35,6 +35,10 @@ def __setitem__(self, key: Hashable, item: Any) -> None:
     def _handle_overrides(self, overrides: Iterable[Override]) -> None:
         for override in overrides:
             logger.debug("Applying override '%s.%s'", override.namespace, override.name)
+            if override.name == "requires":
+                self.data["requires"] = self.data.get("requires", set()) | override.value
+                continue
+
             if override.name in self.data:
                 _apply_override(self.data, override)
                 continue

tests/build/test_generate_config.py

@@ -413,3 +413,27 @@ def test_requires_config_option(program):
 
     assert "MBED_CONF_PLATFORM_STDIO_BAUD_RATE=9600" in config_text
     assert "MBED_LFS_READ_SIZE=64" not in config_text
+
+
+def test_target_requires_config_option(program):
+    create_mbed_app_json(program.root, target_overrides={"*": {"target.requires": ["ble"]}})
+    create_mbed_lib_json(
+        program.mbed_os.root / "bare-metal" / "mbed_lib.json",
+        "ble",
+        target_overrides={"*": {"target.requires": ["platform"]}},
+    )
+    create_mbed_lib_json(
+        program.mbed_os.root / "platform" / "mbed_lib.json", "platform", config={"stdio-baud-rate": {"value": 9600}},
+    )
+    create_mbed_lib_json(
+        program.mbed_os.root / "storage" / "mbed_lib.json",
+        "filesystem",
+        config={"read_size": {"macro_name": "MBED_LFS_READ_SIZE", "value": 64}},
+    )
+
+    generate_config("K64F", "GCC_ARM", program)
+
+    config_text = program.files.cmake_config_file.read_text()
+
+    assert "MBED_CONF_PLATFORM_STDIO_BAUD_RATE=9600" in config_text
+    assert "MBED_LFS_READ_SIZE=64" not in config_text