deviceinfo: add initfs_extra_compression

For configuring the archive compression parameters for the
initramfs-extra archive.

pkgs/deviceinfo/deviceinfo.go

@@ -14,10 +14,11 @@ import (
 )
 
 type DeviceInfo struct {
-	InitfsCompression string
-	MesaDriver        string
-	ModulesInitfs     string
-	UbootBoardname    string
+	InitfsCompression      string
+	InitfsExtraCompression string
+	MesaDriver             string
+	ModulesInitfs          string
+	UbootBoardname         string
 }
 
 func ReadDeviceinfo(file string) (DeviceInfo, error) {

pkgs/deviceinfo/deviceinfo_test.go

@@ -21,6 +21,7 @@ func TestNameToField(t *testing.T) {
 		{"deviceinfo_modules_initfs", "ModulesInitfs"},
 		{"modules_initfs", "ModulesInitfs"},
 		{"deviceinfo_modules_initfs___", "ModulesInitfs"},
+		{"deviceinfo_initfs_extra_compression", "InitfsExtraCompression"},
 	}
 
 	for _, table := range tables {