v4.0.0

• HAL: Added "Listen-Before-Talk" support for Semtech SX1301AP2 Ref Design.
  A description of the feature implementation can be found in
  libloragw/readme.md.
• HAL: Updated FSK RSSI calculation for better linearization
• util_lbt_test: New utility provided for basic "Listen-Before-Talk" testing.
• util_tx_test: Extended to configure and test "LBT" through the HAL.
• Added a reset_lgw.sh script to be used with IoT Starter Kit (v1.0) to reset
  the concentrator through the HOST GPIO pin.