Rewrite command.{c,h} -> usb.{c,h} based on tankbot serial code.
Remove unnecessary plumbing by combining download/monitor.
Move relay mode into timer_status to simplify display.
Rewrite GPS serial parsing and include GPS in status packets.
Remove obsoleted comments
Remove typedefs for consistency.
Length-prefix string messages.
Standardize packet type names.
Increase LCD startup delay, which appears to solve display issues.
Remove an unused #include.
Update datasheets for new hardware.
Rename PPS -> Pulse
Move missing pulse warning to main loop.
Enable brown-out detection at 4.5V.
Disable On-Chip Debugging.
Alert GPS unavailable every 3 seconds.
current_timestamp is accessed from an interrupt.
Count pulses when serial is unavailable in both modes.
Remove duplicated code from pulse interrupt.
WIP: Remove countdown_mode. NEEDS TESTING.