AP_Mount: add MNTx_SYSID param to easier support following another vehicle

[rmackay9]

Similar to Copter's follow mode, AP_Mount should have a SYSID parameter that allows users to more easily specify what vehicle they would like the gimbal to point at when in ROI_SYSID mode.

This request comes from this discussion.

[arikrupnik]

This should probably also add 5: SysId Target as a valid value to MNTx_DEFLT_MODE

[khanasif786]

@rmackay9 @arikrupnik . Do we need to really add a new parameter for this ? I think we just need to change the description of th e MNTx_DEFLT_MODE so that GCS should show 5: SysId Target . and also update the corresponding wiki. and thus the already existing way of targeting a vehicle by using MAV_CMD_DO_SET_ROI_SYSID command will remain same.

[khanasif786]

@rmackay9 New sysID param will introduce an ambiguity when MAV_CMD_DO_SET_ROI_SYSID is sent and then if again parameter is changed in the same flight.

[rmackay9]

@khanasif786,

Re the first question, yes, I totally agree we just need to update the MNTx_DEFLT_MODE parameter description and make sure (in SITL) that AP_Mount really changes to that mode soon after startup.

Re the 2nd question, I think if we have a new parameter it will just be used as the default until a do-set-roi-sysid command arrives. Users may not have an easy way to send a do-set-roi-sysid so having this param would allow them to set it easily... so really it might be better named something like MNTx_SYSID_DEFAULT.