Define default USER_BTN if needed

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

examples/ExternalWakeup/ExternalWakeup.ino

@@ -19,6 +19,10 @@
 volatile int repetitions = 1;
 
 // Pin used to trigger a wakeup
+#ifndef USER_BTN
+#define USER_BTN SYS_WKUP1
+#endif
+
 const int pin = USER_BTN;
 
 void setup() {