Add KairosFS type and detectBoot with FS (#46)

types/fs.go

@@ -0,0 +1,7 @@
+package types
+
+// KairosFS is our interface for methods that need an FS
+// We should try to keep it to a minimum so we can change between backends easily if needed
+type KairosFS interface {
+	ReadFile(filename string) ([]byte, error)
+}