LoRaWAN allow to adapt antenna gain

[hallard]

Summary of changes

Allow to override default antenna gain for LoRaWAN, by default it is setup to 2.15 for all region except US (0.0) this means that whatever antenna you have TX power is always set to 13dBm you have no way to reduce or increase TX power.

Impact of changes

No change by default, allow to adapt TX power depending on antenna gain only by adding a new define

Documentation

To set TX power just add a new macro called LORAPHY_ANTENNA_GAIN with gain (float).

Example if you want to set TX power EU868 to 14dBm just add following in mbed_app.conf

    "macros": [
      "LORAPHY_ANTENNA_GAIN=1.15f"
    ]

Or add following in custom_targets.json

"macros_add": [  
  "LORAPHY_ANTENNA_GAIN=1.15f"
]

PS : I would love adding this parameter in mbed_lib.json of LoRaWAN but since antenna gain is not by default the same for each region, introducing default value of 2.15f would have introduce a breaking change for US915.

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Pull request type

[x] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[x] Feature update (New feature / Functionality change / New API)
[] Major update (Breaking change E.g. Return code change / API behaviour change)

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Test results

[] No Tests required for this change (E.g docs only update)
[x] Covered by existing mbed-os tests (Greentea or Unittest)
[] Tests / results supplied as part of this PR

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Reviewers

@jeromecoutant @0xc0170

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[0xc0170]

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