Accept USB BOOTSEL reboot on the UART firmware

The regular firmware now answers the EP0 vendor INFO and BOOTSEL reboot
requests with the AIO wire protocol, so switch-pico-config reboot bootsel
works when the Pico's USB side is on a PC. Wire constants move to
usb_management_protocol.h; other operations stall.

tests/native_stubs/pico/bootrom.h

@@ -0,0 +1,14 @@
+#pragma once
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+void reset_usb_boot(uint32_t usb_activity_gpio_pin_mask,
+                    uint32_t disable_interface_mask);
+
+#ifdef __cplusplus
+}
+#endif