Targets should have a field to indicate stable version of daplink

[MarceloSalazar]

Targets should have a field to indicate stable version of daplink, for example daplink_version available here:
https://github.com/mbedmicro/mbed/blob/master/hal/targets.json

This would help mbedls tools to display a warning and ask the user to update the firmware (see request here ARMmbed/greentea#148)

[sg-]

This should be managed by a database API where this information is stored, not the target description.