dirs: add features directory

The features directory is designed to store marker files for optional
features that are activated in snapd that need to be checked by
snap-update-ns or other programs that cannot read the state file
directly.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

dirs/dirs.go

@@ -109,6 +109,8 @@ var (
 
 	ErrtrackerDbDir string
 	SysfsDir        string
+
+	FeaturesDir string
 )
 
 const (
@@ -292,4 +294,6 @@ func SetRootDir(rootdir string) {
 
 	ErrtrackerDbDir = filepath.Join(rootdir, snappyDir, "errtracker.db")
 	SysfsDir = filepath.Join(rootdir, "/sys")
+
+	FeaturesDir = filepath.Join(rootdir, snappyDir, "features")
 }