HAL_PX4: enable the FMU PWM pins to be used as GPIO when needed
AP_BoardConfig: board specific config library

used to control board specific parameters, such as PWM output count
Rover: enable AP_BoardConfig
Copter: enable AP_BoardConfig
Plane: enable AP_BoardConfig
AP_Relay: enable up to 4 relay pins
AP_Camera: updates for relay API change
Plane: fixed relay handling

the parameters were badly out of whack!
Plane: fixed servo and relay repeat code
Rover: fixes for AP_Relay API change
Copter: fixes for AP_Relay API change
AP_Relay: added enabled() API
AP_ServoRelayEvents: library for handling servo and relay events
Plane: use AP_ServoRelayEvents library
Rover: converted to AP_ServoRelayEvents
Copter: converted to AP_ServoRelayEvents
Plane: fixed disarmed PWM output