Merge pull request #3708 from fjl/log-letter

cmd/bootnode/main.go

cmd/evm/main.go

cmd/wnode/main.go

eth/protocol_test.go

internal/debug/flags.go

log/format.go

log/logger.go

log/root.go

mobile/init.go

mobile/logger.go

p2p/server_test.go

swarm/network/syncdb_test.go

tests/util.go