stm32/mboot: Move some BSS vars to new section that isn't zeroed out.

Zeroing out data on startup takes time and is not necessary for certain
variables.  So provide a declaration for such variables and use it.

ports/stm32/mboot/fsload.c

@@ -42,7 +42,7 @@ typedef struct _gz_stream_t {
     uint8_t dict[DICT_SIZE];
 } gz_stream_t;
 
-static gz_stream_t gz_stream;
+static gz_stream_t gz_stream SECTION_NOZERO_BSS;
 
 static int gz_stream_read_src(TINF_DATA *tinf) {
     UINT n;

ports/stm32/mboot/main.c

@@ -754,7 +754,7 @@ typedef struct _dfu_state_t {
     uint8_t buf[DFU_XFER_SIZE] __attribute__((aligned(4)));
 } dfu_state_t;
 
-static dfu_state_t dfu_state;
+static dfu_state_t dfu_state SECTION_NOZERO_BSS;
 
 static void dfu_init(void) {
     dfu_state.status = DFU_STATUS_IDLE;
@@ -1070,7 +1070,7 @@ static const USBD_ClassTypeDef pyb_usbdd_class = {
     pyb_usbdd_GetDeviceQualifierDescriptor,
 };
 
-static pyb_usbdd_obj_t pyb_usbdd;
+static pyb_usbdd_obj_t pyb_usbdd SECTION_NOZERO_BSS;
 
 static int pyb_usbdd_detect_port(void) {
     #if MBOOT_USB_AUTODETECT_PORT
@@ -1096,6 +1096,7 @@ static int pyb_usbdd_detect_port(void) {
 
 static void pyb_usbdd_init(pyb_usbdd_obj_t *self, int phy_id) {
     self->started = false;
+    self->tx_pending = false;
     USBD_HandleTypeDef *usbd = &self->hUSBDDevice;
     usbd->id = phy_id;
     usbd->dev_state = USBD_STATE_DEFAULT;

ports/stm32/mboot/mboot.h

@@ -27,6 +27,9 @@
 #include <stdint.h>
 #include <stddef.h>
 
+// Use this to tag global static data in RAM that doesn't need to be zeroed on startup
+#define SECTION_NOZERO_BSS __attribute__((section(".nozero_bss")))
+
 #define ELEM_DATA_START (&_estack)
 #define ELEM_DATA_MAX (ELEM_DATA_START + 1024)
 

ports/stm32/mboot/stm32_generic.ld

@@ -53,18 +53,25 @@ SECTIONS
         _edata = .;
     } >RAM AT> FLASH_BL
 
-    /* Uninitialized data section */
+    /* Zeroed-out data section */
     .bss :
     {
         . = ALIGN(4);
         _sbss = .;
         *(.bss*)
         *(COMMON)
-
         . = ALIGN(4);
         _ebss = .;
     } >RAM
 
+    /* Uninitialized data section */
+    .nozero_bss (NOLOAD) :
+    {
+        . = ALIGN(4);
+        *(.nozero_bss*)
+        . = ALIGN(4);
+    } >RAM
+
     /* this just checks there is enough RAM for the stack */
     .stack :
     {