Modify linux flasher build to compile for ARM

This is needed for the flasher to be usable on the Raspberry Pi (with a 32-bit userspace). Also, switch to using a submodule for the firmware-flasher repo to follow Git and GitLab CI best-practices.

I don't have a Pi with a 32-bit OS installed, so I was not able to test this binary. Someone else should before this is merged.