Build cleanup

[luke-jr]

Looking into possibly using this as a base for a free software firmware replacement on a M3-based device, but found it a bit ugly to add a new device. This refactor moves device-specific configs to their own file, and skips building sources that aren't configured (by doing configuration at the makefile level)

[microbuilder]

Luke ... thanks for the submission. I'm not able to test this at the moment since I'm away for the next two weeks, but I'll try it out when I have some real HW in front of me.