MOUNT: Add a mount2 retract

[muramura]

Rovers and other vehicles sometimes have cameras mounted on both the front and rear.

[rmackay9]

Hi @muramura,

We discussed at this week's EU dev call and to save flash could you please add a new method called, "do_aux_function_retract_mount()" that takes a "ch_flag" and a "uint8_t instance" which is set to 0 or 1 depending upon whether we're handling RETRACT_MOUNT1 or RETRACT_MOUNT2? then use this new method to handle both of the RETRACT_MOUNT1/2 events.

[muramura]

@rmackay9 san. I understand. I have made the changes.

[muramura]

@IamPete1 san. I made changes.

[muramura]

@IamPete1 san. I agreed.