Startup cmd `bera-geth` should correctly default to Berachain mainnet

[calbera]

Code locations to fix (among others):

• SetEthConfig (cmd/utils/flags.go)
• loadBaseConfig (cmd/bera-geth/config.go)
  • eth/ethconfig/config.go Defaults should correctly set Berachain mainnet chain config as the Genesis if its empty/nil

Will help in situations mainly for Mainnet and Bepolia where operators can upgrade their binary without re-initing the genesis config file