fix: fail to build usr

[AndreCostaaa]

Fixes #102

• There were some unused variables, returns on void function, missing returns on non-void function that are now fixed

• Changed the CMakelists found in usr so we can pass some CFLAGS from the command line and modified the CI to pass -Werror when building

• Refactored the old build-virt32 and build-virt64 by using a matrix making it easier to add new so3 configs to the CI

Also i've noticed that building usr in release mode fails in both arm and aarch64 toolchains. Not really sure it's something worth fixing for now at least so i'm not adding that option to the CI

[daniel-rossier]

Just one thing, normally creating the branch from the issue should prefix with the number of issue. It seems the branch "fix-fail-to-build-usr" doesn't have such a prefix.

[AndreCostaaa]

Just one thing, normally creating the branch from the issue should prefix with the number of issue. It seems the branch "fix-fail-to-build-usr" doesn't have such a prefix.

Yes, i'm used to removing the number as i feel it doesn't really add much having it on the branch name

Just a personal opinion of course, will make sure to leave it for my next contributions here 😄

[daniel-rossier]

I guess it's really easy to mention a specific branch by its number rather than its title that nobody really remembers most of time. So it helps :-)