Powers off eink backlights before shutdown

- t-echo boards in particular need this
- follows the existing defines structure

src/Power.cpp

@@ -234,6 +234,9 @@ bool Power::setup()
 void Power::shutdown()
 {
 
+#if defined(USE_EINK)
+    digitalWrite(PIN_EINK_EN, LOW); //power off backlight first
+#endif
 
 #ifdef HAS_PMU
     DEBUG_MSG("Shutting down\n");