o Implement aux with M42 Pxx Sxx. Controls both aux pins
  on Bumps.
  E.g.
    M42 P0 S1  ; switch on Aux 1
    M42 P1 S1  ; switch on Aux 2
    M42 P1 S0  ; switch off Aux 2
 This is synchronous, i.e. will be active with the next
 axis move command.