rp2/modmachine: Allow boards to provide custom bootloader code.

And expose the machine_bootloader() C function so it can be used elsewhere.

ports/rp2/modmachine.c

@@ -81,11 +81,13 @@ STATIC mp_obj_t machine_reset_cause(void) {
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_0(machine_reset_cause_obj, machine_reset_cause);
 
-STATIC mp_obj_t machine_bootloader(void) {
+NORETURN mp_obj_t machine_bootloader(size_t n_args, const mp_obj_t *args) {
+    MICROPY_BOARD_ENTER_BOOTLOADER(n_args, args);
     reset_usb_boot(0, 0);
-    return mp_const_none;
+    for (;;) {
+    }
 }
-MP_DEFINE_CONST_FUN_OBJ_0(machine_bootloader_obj, machine_bootloader);
+STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_bootloader_obj, 0, 1, machine_bootloader);
 
 STATIC mp_obj_t machine_freq(size_t n_args, const mp_obj_t *args) {
     if (n_args == 0) {

ports/rp2/modmachine.h

@@ -18,5 +18,6 @@ void machine_pin_deinit(void);
 void machine_i2s_init0(void);
 
 struct _machine_spi_obj_t *spi_from_mp_obj(mp_obj_t o);
+NORETURN mp_obj_t machine_bootloader(size_t n_args, const mp_obj_t *args);
 
 #endif // MICROPY_INCLUDED_RP2_MODMACHINE_H

ports/rp2/mpconfigport.h

@@ -130,6 +130,10 @@
 #define mp_type_textio mp_type_vfs_lfs2_textio
 #endif
 
+#ifndef MICROPY_BOARD_ENTER_BOOTLOADER
+#define MICROPY_BOARD_ENTER_BOOTLOADER(nargs, args)
+#endif
+
 #if MICROPY_PY_NETWORK
 #define NETWORK_ROOT_POINTERS               mp_obj_list_t mod_network_nic_list;
 #else