Plain RAK4631 should not compile EInk and TFT display code (#8811)

* Plain RAK4631 should not compile EInk and TFT display code

* Add USE_TFTDISPLAY to variant files.

* Derp

* Undo the platformio.ini changes to heltec_v4

* Drop unneeded src_filter lines

---------

Co-authored-by: Jonathan Bennett <jbennett@incomsystems.biz>
Co-authored-by: Jason P <applewiz@mac.com>

src/configuration.h

@@ -36,6 +36,29 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 /* Offer chance for variant-specific defines */
 #include "variant.h"
 
+// -----------------------------------------------------------------------------
+// Display feature overrides
+// -----------------------------------------------------------------------------
+
+// Allow build environments to opt-in explicitly to the E-Ink UI stack while
+// keeping headless targets slim by default. Existing variants that already
+// define USE_EINK continue to work without additional flags.
+#ifndef MESHTASTIC_USE_EINK_UI
+#ifdef USE_EINK
+#define MESHTASTIC_USE_EINK_UI 1
+#else
+#define MESHTASTIC_USE_EINK_UI 0
+#endif
+#endif
+
+#if MESHTASTIC_USE_EINK_UI
+#ifndef USE_EINK
+#define USE_EINK
+#endif
+#else
+#undef USE_EINK
+#endif
+
 // -----------------------------------------------------------------------------
 // Version
 // -----------------------------------------------------------------------------
@@ -371,6 +394,9 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #ifndef HAS_BLUETOOTH
 #define HAS_BLUETOOTH 0
 #endif
+#ifndef USE_TFTDISPLAY
+#define USE_TFTDISPLAY 0
+#endif
 
 #ifndef HW_VENDOR
 #error HW_VENDOR must be defined