Revert "Delete external_lcd.cpp"

This reverts commit a169d71d30.

src/external_lcd.cpp

@@ -0,0 +1,5 @@
+#include <external_lcd.hpp>
+void ExternalLCD::callTriggerFunction() {
+    uint8_t touch_hex = Serial2.read();
+    external_lcd_callback(touch_hex);
+}