Update LED usage

[mossmann]

Proposed plan:

• LED A: USB switch is in Apollo/Saturn-V mode (see PROGRAM button doesn't toggle USB switch #38)
• LED B: FPGA is being held offline
• LED C: Saturn-V heartbeat
• LED D: fault, e.g. FPGA failed to configure from flash
• LED E: Apollo heartbeat

[proposal replaced by "Alternative scheme" in comment below]

All LEDs would change function temporarily if a pattern other than idle is in use. The idle pattern would restore the indications above.

[martinling]

Personally I would love it if the LEDs would stop flashing when nothing is happening (e.g. Apollo is idle and a bitstream is running OK). To me, flashing means something is wrong and it wants attention.

[mossmann]

Alternative scheme @martinling and I came up with:

• LED A: power indication (always on in Apollo)
• LED B: FPGA allowed online (indicates PROGRAM toggle)
• LED C: FPGA has requested CONTROL port
• LED D: USB switched to FPGA
• LED E: flashing patterns (e.g. fault indication)

All LEDs would be off in Saturn-V mode except C flashes. This requires no change to Saturn-V.