ardupilot: added VEHICLE_MODE message

[tridge]

this will allow for the creation of custom flight modes in lua scripts

[auturgy]

Could this be considered in context of https://github.com/mavlink/rfcs/pull/17/files where a proposal for a somewhat similar message:

    <message id="435" name="AVAILABLE_MODES">
      <description>Get information about a particular flight modes.
        The message can be enumerated or requested for a particular mode using MAV_CMD_REQUEST_MESSAGE.
        Specify 0 in param2 to request that the message is emitted for all available modes or the specific index for just one mode.
        The modes must be available/settable for the current vehicle/frame type.
        Each modes should only be emitted once (even if it is both standard and custom).
      </description>
      <field type="uint8_t" name="number_modes">The total number of available modes for the current vehicle type.</field>
      <field type="uint8_t" name="mode_index">The current mode index within number_modes, indexed from 1.</field>
      <field type="uint8_t" name="standard_mode" enum="MAV_STANDARD_MODE">Standard mode.</field>
      <field type="uint8_t" name="base_mode" enum="MAV_MODE_FLAG" display="bitmask">System mode bitmap.</field>
      <field type="uint32_t" name="custom_mode">A bitfield for use for autopilot-specific flags</field>
      <field type="char[50]" name="mode_name">Name of custom mode, without null termination character. Should be omitted for standard modes.</field>
    </message>

[rmackay9]

I wonder if "MODE_DESCRIPTION" might be a better name. This message's purpose is not to provide the current mode but rather to give information about a mode.

Otherwise it looks OK I think.

[rmackay9]

@auturgy's message could cover the same requirement if the "short_name" field was added to it... I think

[auturgy]

@hamishwillee fyi

[hamishwillee]

Perhaps stupid questions but:

• What's the point of this short name - it's only 4 chars?
• What's the point of the long name for your use case? I.e. a lua script is going to need to know what modes it is dealing with right or are you presenting this to the user?
• RFC uses 50 chars for the name while you have granted 20. Are we wasting bits in the RFC message?

There is certainly a lot of overlap so assuming we go for the standard modes would be better to use that.