HAL_ChibiOS: added a delete_next thread field for cleanup

allows for cleanup of web server threads in bootloader

libraries/AP_HAL_ChibiOS/hwdef/common/chconf.h

@@ -708,8 +708,10 @@ extern "C" {
  * @brief   Threads descriptor structure extension.
  * @details User fields added to the end of the @p thread_t structure.
  */
+#ifndef CH_CFG_THREAD_EXTRA_FIELDS
 #define CH_CFG_THREAD_EXTRA_FIELDS                                          \
   /* Add threads custom fields here.*/
+#endif
 
 /**
  * @brief   Threads initialization hook.

libraries/AP_HAL_ChibiOS/hwdef/include/network_bootloader.inc

@@ -20,3 +20,5 @@ EXT_FLASH_SIZE_MB 0
 
 # don't combine sdcard bl and network bl
 undef AP_BOOTLOADER_FLASH_FROM_SD_ENABLED
+
+define CH_CFG_THREAD_EXTRA_FIELDS struct ch_thread *delete_next;