Define BUTTON_PIN as -1 for RP2040-lora (#5574)

The previous approach of undef'ing meant that it was impossible
for users to change the button pin in the apps.

Fixes https://github.com/meshtastic/firmware/issues/5566

variants/rp2040-lora/variant.h

@@ -15,7 +15,7 @@
 // rxd = 9
 
 #define EXT_NOTIFY_OUT 22
-#undef BUTTON_PIN // Pin 17 used for antenna switching via DIO4
+#define BUTTON_PIN -1 // Pin 17 used for antenna switching via DIO4
 
 #define LED_PIN PIN_LED
 
@@ -57,4 +57,4 @@
 #define SX126X_DIO2_AS_RF_SWITCH // Antenna switch CTRL
 #define SX126X_RXEN LORA_DIO4    // Antenna switch !CTRL via GPIO17
 // #define SX126X_DIO3_TCXO_VOLTAGE 1.8
-#endif
+#endif