Mainboard revision B (manufacturing/assembly changes)

[fjmilens3]

Bumps the schematic and PCB revisions to Rev B.

Fixes known issues with the original layout:

• Move RCA connectors to better align with punchout on case.

Includes PCB revision recommendations from @roscopeco / Really Old-School Company for easier manufacturing/assembly:

• Change IC socket footprints to long pads for easier hobbyist soldering.
• Add do-not-populate debug input for DC power (J10).
• Change resistor footprints for easier assembly. (Didn't do this.)