lnd: fix golint warning which requires to add additional comments

lnwallet/btcwallet/config.go

@@ -7,7 +7,10 @@ import (
 	"github.com/roasbeef/btcd/wire"
 	"github.com/roasbeef/btcutil"
 
-	// TODO(roasbeef) add comment which justifying this import.
+	// This is required to register bdb as a valid walletdb driver. In the
+	// init function of the package, it registers itself. The import is used
+	// to activate the side effects w/o actually binding the package name to
+	// a file-level variable.
 	_ "github.com/roasbeef/btcwallet/walletdb/bdb"
 )