Restart Sidecar sequencer task if mainboard controller is reset

If some external event or entity resets the mainboard controller FPGA after the sequencer task has started it'll not continue to make forward progress. Consider the following log:

humility: ring buffer drv_sidecar_seq_server::__RINGBUF in sequencer:
 NDX LINE      GEN    COUNT PAYLOAD
   0  442        1        1 FpgaInit
   1  485        1        1 SkipLoadingBitstream
   2  495        1        1 MainboardControllerId(0x1de5bae)
   3  509        1        1 MainboardControllerChecksum(0x6461823f)
   4  541        1        1 MainboardControllerVersion(0x83)
   5  542        1        1 MainboardControllerSha(0x7b53e239)
   6  543        1        1 FpgaInitComplete
   7   25        1        1 LoadingClockConfiguration
   8  564        1        1 ClockConfigurationComplete
   9  568        1        1 FrontIOBoardPresent
  10   58        1        1 FrontIOControllerIdent { fpga_id: 0x0, ident: 0x1deaa55 }
  11   64        1        1 FrontIOControllerChecksum { fpga_id: 0x0, checksum: [ 0x72, 0xa5, 0x6b, 0xec ], expected: [ 0x72, 0xa5, 0x6b, 0xec ] }
  12   78        1        1 SkipLoadingFrontIOControllerBitstream { fpga_id: 0x0 }
  13   58        1        1 FrontIOControllerIdent { fpga_id: 0x1, ident: 0x1deaa55 }
  14   64        1        1 FrontIOControllerChecksum { fpga_id: 0x1, checksum: [ 0x72, 0xa5, 0x6b, 0xec ], expected: [ 0x72, 0xa5, 0x6b, 0xec ] }
  15   78        1        1 SkipLoadingFrontIOControllerBitstream { fpga_id: 0x1 }
  16  581        1        1 FrontIOVsc8562Ready
  17  121        1     5557 TofinoSequencerTick(Disabled, A2, None)
  18  402        1     5579 TofinoSequencerError(FpgaError)

If a TofinoSequencerError is encountered the sequencer should check the device state of the mainboard controller and if it finds the device AwaitingBitstream it should restart itself. The FPGA will have its IOs floating, so nothing on the board works anyway and we may as well restart the task.

Start at the sequencer task's handling of TofinoSequencerError and inspect how it reads the mainboard controller device state. Confirm the behavior using the supplied ring-buffer scenario: when the state is AwaitingBitstream, the sequencer should restart; otherwise it should continue its existing handling.