lib/utils/interrupt_char: Invalidate interrupt char at start up.

Otherwise mp_interrupt_char will have a value of zero on start up (because
it's in the BSS) and a KeyboardInterrupt may be raised during start up.
For example this can occur if there is a UART attached to the REPL which
sends spurious null bytes when the device turns on.

lib/utils/interrupt_char.c

@@ -29,7 +29,7 @@
 
 #if MICROPY_KBD_EXCEPTION
 
-int mp_interrupt_char;
+int mp_interrupt_char = -1;
 
 void mp_hal_set_interrupt_char(int c) {
     if (c != -1) {