Implement M400 (wait for buffer to be executed).
Implement M400 (wait for buffer to be executed).

johnkingsley/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,ambrop72/aprinter,johnkingsley/aprinter,johnkingsley/aprinter,fixator/aprinter,johnkingsley/aprinter,fixator/aprinter,fixator/aprinter,johnkingsley/aprinter,fixator/aprinter,fixator/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,ambrop72/aprinter,fixator/aprinter,ambrop72/aprinter