stm32: Use a separate symbol name for the bootloader state pointer.

Prerequisite for enabling Link Time Optimisation.

The _bl_state address is the same as _estack, but _estack is referred to as
a uint32_t elsewhere in the code. LTO doesn't like it when the same symbol
has two different types.

Signed-off-by: Angus Gratton <gus@projectgus.com>

ports/stm32/boards/common_bss_heap_stack.ld

@@ -26,3 +26,7 @@
     . = . + _minimum_stack_size;
     . = ALIGN(4);
 } >RAM
+
+/* _bl_state symbol is used by MICROPY_HW_ENTER_BOOTLOADER_VIA_RESET, this is
+   the end of stack address but is accessed as a different type. */
+_bl_state = _estack;

ports/stm32/powerctrl.c

@@ -78,7 +78,7 @@
 // Location in RAM of bootloader state (just after the top of the stack).
 // STM32H7 has ECC and writes to RAM must be 64-bit so they are fully committed
 // to actual SRAM before a system reset occurs.
-#define BL_STATE_PTR                ((uint64_t *)&_estack)
+#define BL_STATE_PTR                ((uint64_t *)&_bl_state)
 #define BL_STATE_KEY                (0x5a5)
 #define BL_STATE_KEY_MASK           (0xfff)
 #define BL_STATE_KEY_SHIFT          (32)
@@ -87,7 +87,7 @@
 #define BL_STATE_GET_REG(s)         ((s) & 0xffffffff)
 #define BL_STATE_GET_KEY(s)         (((s) >> BL_STATE_KEY_SHIFT) & BL_STATE_KEY_MASK)
 #define BL_STATE_GET_ADDR(s)        (((s) >> BL_STATE_KEY_SHIFT) & ~BL_STATE_KEY_MASK)
-extern uint64_t _estack[];
+extern uint64_t _bl_state[];
 #endif
 
 static inline void powerctrl_disable_hsi_if_unused(void) {