Additional methods PIOS_Servo_Enable() and PIOS_Servo_Disable() are needed for ESC passthrough feature to be able to gain access to timers and gpio peripherals used by servo output pins.
pios_servo should not be pios_tim client anymore, to work around pios_tim callback bug (callbacks for multiple clients on same timer/channel combo are invoked for 1st registered client only). Also, servo driver is not using timer callbacks at all, so there is very little functionality from pios_tim that it uses - which is just gpio initialization. Gpio initialization needs to be done in PIOS_Servo_Enable() anyway.
pios_servo.c is mostly identical for F1 & F4, therefore it should live in pios/common instead of having multiple personalities disorder like now.