Add MIN_LORA_TX_POWER build flag for companion radio

[csrutil]

Summary

• Add a MIN_LORA_TX_POWER build flag for the companion radio. It sets the lower TX power limit, as MAX_LORA_TX_POWER sets the upper limit.
• The default is -9, which is the value that was hard-coded before. Builds that do not set the flag do not change.
• Use the flag in the prefs clamp in MyMesh::begin() and in the range check for CMD_SET_RADIO_TX_POWER.