add support for build tag to allow to disable some features

The following build tags are available:

- "nogcs", disable Google Cloud Storage backend
- "nos3", disable S3 Compabible Object Storage backends
- "nobolt", disable Bolt data provider
- "nomysql", disable MySQL data provider
- "nopgsql", disable PostgreSQL data provider
- "nosqlite", disable SQLite data provider
- "noportable", disable portable mode

cmd/portable.go

@@ -1,3 +1,5 @@
+// +build !noportable
+
 package cmd
 
 import (
@@ -14,6 +16,7 @@ import (
 	"github.com/drakkan/sftpgo/dataprovider"
 	"github.com/drakkan/sftpgo/service"
 	"github.com/drakkan/sftpgo/sftpd"
+	"github.com/drakkan/sftpgo/utils"
 	"github.com/drakkan/sftpgo/vfs"
 )
 
@@ -138,6 +141,8 @@ Please take a look at the usage below to customize the serving parameters`,
 )
 
 func init() {
+	utils.AddFeature("+portable")
+
 	portableCmd.Flags().StringVarP(&directoryToServe, "directory", "d", ".",
 		"Path to the directory to serve. This can be an absolute path or a path relative to the current directory")
 	portableCmd.Flags().IntVarP(&portableSFTPDPort, "sftpd-port", "s", 0, "0 means a random non privileged port")