esp32/mpconfigport: Don't hang when machine.bootloader isn't supported.

Currently only the Arduino Nano ESP32 defines a machine.bootloader handler
for ESP32.  All other boards will intentionally hang.

There is no error message, nor is a NotImplementedError raised.  There's no
indication if Micropython has crashed, or if the bootloader was entered but
USB is not working, which is a real problem the ESP32 bootloader has.  It's
not possible escape from this hang with ^C or any other means besides
physical access to the reset pin or the ability to cycle power.

Change this to only define an implementation of machine.bootloader() when
there is a handler for it.

Signed-off-by: Trent Piepho <tpiepho@gmail.com>

ports/esp32/modmachine.c

@@ -220,11 +220,13 @@ static mp_int_t mp_machine_reset_cause(void) {
     }
 }
 
+#ifdef MICROPY_BOARD_ENTER_BOOTLOADER
 NORETURN void mp_machine_bootloader(size_t n_args, const mp_obj_t *args) {
     MICROPY_BOARD_ENTER_BOOTLOADER(n_args, args);
     for (;;) {
     }
 }
+#endif
 
 void machine_init(void) {
     is_soft_reset = 0;

ports/esp32/mpconfigport.h

@@ -115,7 +115,6 @@
 #define MICROPY_PY_MACHINE                  (1)
 #define MICROPY_PY_MACHINE_INCLUDEFILE      "ports/esp32/modmachine.c"
 #define MICROPY_PY_MACHINE_BARE_METAL_FUNCS (1)
-#define MICROPY_PY_MACHINE_BOOTLOADER       (1)
 #define MICROPY_PY_MACHINE_DISABLE_IRQ_ENABLE_IRQ (1)
 #define MICROPY_PY_MACHINE_ADC              (1)
 #define MICROPY_PY_MACHINE_ADC_INCLUDEFILE  "ports/esp32/machine_adc.c"
@@ -269,8 +268,10 @@ typedef long mp_off_t;
 #define MICROPY_HW_ENABLE_MDNS_RESPONDER    (1)
 #endif
 
-#ifndef MICROPY_BOARD_ENTER_BOOTLOADER
-#define MICROPY_BOARD_ENTER_BOOTLOADER(nargs, args)
+#ifdef MICROPY_BOARD_ENTER_BOOTLOADER
+#define MICROPY_PY_MACHINE_BOOTLOADER       (1)
+#else
+#define MICROPY_PY_MACHINE_BOOTLOADER       (0)
 #endif
 
 #ifndef MICROPY_BOARD_STARTUP