Add optional external GPS for TTGO Lora V2

per this request: https://meshtastic.discourse.group/t/gps-pin-assignments-for-custom-boards/501/9?u=geeksville

src/configuration.h

@@ -144,8 +144,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #define I2C_SDA 21
 #define I2C_SCL 22
 
-#define BUTTON_PIN 38       // The middle button GPIO on the T-Beam
+#define BUTTON_PIN 38     // The middle button GPIO on the T-Beam
-#define BUTTON_PIN_ALT 13   // Alternate GPIO for an external button if needed
+#define BUTTON_PIN_ALT 13 // Alternate GPIO for an external button if needed
 
 #ifndef USE_JTAG
 #define RESET_GPIO 14
@@ -236,6 +236,11 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 // This string must exactly match the case used in release file names or the android updater won't work
 #define HW_VENDOR "ttgo-lora32-v2"
 
+#undef GPS_RX_PIN
+#undef GPS_TX_PIN
+#define GPS_RX_PIN 36
+#define GPS_TX_PIN 13 // per @eugene
+
 #define I2C_SDA 21 // I2C pins for this board
 #define I2C_SCL 22