docs(mavlink.md): add CPM_mavlink_SOURCE variable example

[iftahnaf]

Update MAVLink.md - Remove Unused CMake Variable

Description

Remove CPM_mavlink_SOURCE variable from mavlink.md documentation as it is not used anywhere.

Test Steps

Checklist:

• Review Contribution Guidelines.
• Review Code of Conduct.
• I have tested my changes.

Related Issue

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[HTRamsey]

This is actually read by CPM, you can use the same variable format to override any of the dependencies

[iftahnaf]

@HTRamsey Thanks for the quick response.
Apologies for missing it.
I've added a usage example, which may be helpful.

[dakejahl]

This isn't working for me, am I missing something?

In file included from /home/jake/code/jake/qgroundcontrol/src/AutoPilotPlugins/APM/APMCameraComponent.cc:11:
/home/jake/code/jake/qgroundcontrol/src/Vehicle/Vehicle.h:639:37: error: ‘MAV_CMD’ has not been declared

# MAVLink
# set(QGC_MAVLINK_GIT_REPO "https://github.com/mavlink/c_library_v2.git" CACHE STRING "URL to MAVLink Git Repo")
# set(QGC_MAVLINK_GIT_TAG "19f9955598af9a9181064619bd2e3c04bd2d848a" CACHE STRING "Tag of MAVLink Git Repo")
set(CPM_mavlink_SOURCE "/home/jake/code/jake/PX4-Autopilot/build/ark_fpv_default/mavlink")