[detect] Print a warning if multiple devices are mounted to the same mount point

[bridadan]

This came up during #363. It may be helpful to users to print a warning if multiple devices share a mount point. Linux lets you do this but it will cause Mbed LS to give bogus results. Typically multiple Mbed devices sharing a mount point is a mistake anyway, so this would give users a helpful reminder.

[cmonr]

Why would this happen to begin with?
[Mirrored to Jira]

[bridadan]

If you were managing your mounts manually in linux (no automounter) and you mounted two devices to the same mount point.
[Mirrored to Jira]

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOCUSTRIA-748

[ciarmcom]

Thank you for raising this issue. Please note we have updated our policies and
now only defects should be raised directly in GitHub. Going forward questions and
enhancements will be considered in our forums, https://forums.mbed.com/ . If this
issue is still relevant please re-raise it there.
This GitHub issue will now be closed.