sys/riotboot: add a warning about the size of riotboot_flashwrite_t

On e.g. samd5x FLASHPAGE_SIZE is 8k (4k on kinetis, stm32w, nrf52)
so placing this struct on the stack will almost always cause a stack
overflow.

sys/include/riotboot/flashwrite.h

@@ -59,6 +59,9 @@ extern "C" {
 
 /**
  * @brief   firmware update state structure
+ *
+ * @note    @ref FLASHPAGE_SIZE can be very large on some platforms, don't place
+ *          this struct on the stack or increase the thread stack size accordingly.
  */
 typedef struct {
     int target_slot;                        /**< update targets this slot     */