stm32f1: ubshid: add re-enumeration forcing.

This makes the demo reliably work on "generic" "other" style f1 boards
out of the box.

examples/stm32/f1/other/usb_hid/usbhid.c

@@ -279,6 +279,21 @@ int main(void)
 	rcc_clock_setup_in_hsi_out_48mhz();
 
 	rcc_periph_clock_enable(RCC_GPIOA);
+	/*
+	 * This is a somewhat common cheap hack to trigger device re-enumeration
+	 * on startup.  Assuming a fixed external pullup on D+, (For USB-FS)
+	 * setting the pin to output, and driving it explicitly low effectively
+	 * "removes" the pullup.  The subsequent USB init will "take over" the
+	 * pin, and it will appear as a proper pullup to the host.
+	 * The magic delay is somewhat arbitrary, no guarantees on USBIF
+	 * compliance here, but "it works" in most places.
+	 */
+	gpio_set_mode(GPIOA, GPIO_MODE_OUTPUT_2_MHZ,
+		GPIO_CNF_OUTPUT_PUSHPULL, GPIO12);
+	gpio_clear(GPIOA, GPIO12);
+	for (unsigned i = 0; i < 800000; i++) {
+		__asm__("nop");
+	}
 
 	usbd_dev = usbd_init(&st_usbfs_v1_usb_driver, &dev_descr, &config, usb_strings, 3, usbd_control_buffer, sizeof(usbd_control_buffer));
 	usbd_register_set_config_callback(usbd_dev, hid_set_config);