[build] force -Wreturn-type to always trigger an error

[avtolstoy]

Problem

Starting with GCC 9 not returning a value from a function declared to return non-void type will cause a hardfault. This is easily overlooked in applications where warnings are not promoted to errors by defaults.

Solution

Force -Wreturn-type to always trigger an error even if building applications where warnings are not promoted to errors.

Steps to Test

Test app should fail to build with an error.

Example App

int test() {
}

/* executes once at startup */
void setup() {
	Log.info("%d", test());
}

/* executes continuously after setup() runs */
void loop() {

}

References

N/A

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)