stmhal: Put a USB structure in ROM; GC doesn't scan the data segment.

stmhal/gccollect.c

@@ -21,10 +21,9 @@ void gc_collect(void) {
     // start the GC
     gc_collect_start();
 
-    // scan everything in RAM before the heap
-    // this includes the data and bss segments
-    // TODO possibly don't need to scan data, since all pointers should start out NULL and be in bss
-    gc_collect_root((void**)&_ram_start, ((uint32_t)&_ebss - (uint32_t)&_ram_start) / sizeof(uint32_t));
+    // We need to scan everything in RAM that can hold a pointer.
+    // The data segment is used, but should not contain pointers, so we just scan the bss.
+    gc_collect_root((void**)&_sbss, ((uint32_t)&_ebss - (uint32_t)&_sbss) / sizeof(uint32_t));
 
     // get the registers and the sp
     machine_uint_t regs[10];

stmhal/usb.c

@@ -28,7 +28,7 @@ void pyb_usb_dev_init(usb_device_mode_t mode, usb_storage_medium_t medium) {
         } else {
             USBD_SelectMode(USBD_MODE_CDC_HID);
         }
-        USBD_Init(&hUSBDDevice, &VCP_Desc, 0);
+        USBD_Init(&hUSBDDevice, (USBD_DescriptorsTypeDef*)&VCP_Desc, 0);
         USBD_RegisterClass(&hUSBDDevice, &USBD_CDC_MSC_HID);
         USBD_CDC_RegisterInterface(&hUSBDDevice, (USBD_CDC_ItfTypeDef*)&USBD_CDC_fops);
 #if MICROPY_HW_HAS_SDCARD

stmhal/usbd_desc.h

-extern USBD_DescriptorsTypeDef VCP_Desc;
-extern USBD_DescriptorsTypeDef MSC_Desc;
+extern const USBD_DescriptorsTypeDef VCP_Desc;

stmhal/usbd_desc_cdc_msc.c

@@ -59,7 +59,7 @@ uint8_t *USBD_VCP_USRStringDesc (USBD_SpeedTypeDef speed, uint8_t idx, uint16_t
 #endif /* USB_SUPPORT_USER_STRING_DESC */  
 
 /* Private variables ---------------------------------------------------------*/
-USBD_DescriptorsTypeDef VCP_Desc = {
+const USBD_DescriptorsTypeDef VCP_Desc = {
   USBD_VCP_DeviceDescriptor,
   USBD_VCP_LangIDStrDescriptor, 
   USBD_VCP_ManufacturerStrDescriptor,