nrf/modmachine: Add machine.bootloader() and make it customisable.

ports/nrf/modules/machine/modmachine.c

@@ -153,6 +153,13 @@ STATIC mp_obj_t machine_soft_reset(void) {
 }
 MP_DEFINE_CONST_FUN_OBJ_0(machine_soft_reset_obj, machine_soft_reset);
 
+NORETURN mp_obj_t machine_bootloader(size_t n_args, const mp_obj_t *args) {
+    MICROPY_BOARD_ENTER_BOOTLOADER(n_args, args);
+    for (;;) {
+    }
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_bootloader_obj, 0, 1, machine_bootloader);
+
 STATIC mp_obj_t machine_lightsleep(void) {
     __WFE();
     return mp_const_none;
@@ -196,6 +203,7 @@ STATIC const mp_rom_map_elem_t machine_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR_info),               MP_ROM_PTR(&machine_info_obj) },
     { MP_ROM_QSTR(MP_QSTR_reset),              MP_ROM_PTR(&machine_reset_obj) },
     { MP_ROM_QSTR(MP_QSTR_soft_reset),         MP_ROM_PTR(&machine_soft_reset_obj) },
+    { MP_ROM_QSTR(MP_QSTR_bootloader),         MP_ROM_PTR(&machine_bootloader_obj) },
     { MP_ROM_QSTR(MP_QSTR_enable_irq),         MP_ROM_PTR(&machine_enable_irq_obj) },
     { MP_ROM_QSTR(MP_QSTR_disable_irq),        MP_ROM_PTR(&machine_disable_irq_obj) },
     { MP_ROM_QSTR(MP_QSTR_sleep),              MP_ROM_PTR(&machine_lightsleep_obj) },

ports/nrf/modules/machine/modmachine.h

@@ -38,5 +38,6 @@ MP_DECLARE_CONST_FUN_OBJ_VAR_BETWEEN(machine_info_obj);
 MP_DECLARE_CONST_FUN_OBJ_0(machine_reset_obj);
 MP_DECLARE_CONST_FUN_OBJ_0(machine_lightsleep_obj);
 MP_DECLARE_CONST_FUN_OBJ_0(machine_deepsleep_obj);
+NORETURN mp_obj_t machine_bootloader(size_t n_args, const mp_obj_t *args);
 
 #endif // __MICROPY_INCLUDED_NRF5_MODMACHINE_H__

ports/nrf/mpconfigport.h

@@ -371,6 +371,10 @@ extern const struct _mp_obj_module_t music_module;
 #define MICROPY_BOARD_STARTUP()
 #endif
 
+#ifndef MICROPY_BOARD_ENTER_BOOTLOADER
+#define MICROPY_BOARD_ENTER_BOOTLOADER(nargs, args)
+#endif
+
 #ifndef MICROPY_BOARD_EARLY_INIT
 #define MICROPY_BOARD_EARLY_INIT()
 #endif