mesh: configurable advertiser stack size

Add a configurable option for advertiser thread stack size

this is port of 9cc884a0d3c7a8ac4ddf476d714435f5a52e2016

nimble/host/mesh/src/adv.c

@@ -44,8 +44,7 @@ static s32_t adv_int_min =  ADV_INT_DEFAULT_MS;
  * an increased call stack whenever it's used.
  */
 #if MYNEWT
-#define ADV_STACK_SIZE 768
-OS_TASK_STACK_DEFINE(g_blemesh_stack, ADV_STACK_SIZE);
+OS_TASK_STACK_DEFINE(g_blemesh_stack, MYNEWT_VAL(BLE_MESH_ADV_STACK_SIZE));
 struct os_task adv_task;
 #endif
 
@@ -333,7 +332,7 @@ void bt_mesh_adv_init(void)
 #if MYNEWT
 	os_task_init(&adv_task, "mesh_adv", mesh_adv_thread, NULL,
 	             MYNEWT_VAL(BLE_MESH_ADV_TASK_PRIO), OS_WAIT_FOREVER,
-	             g_blemesh_stack, ADV_STACK_SIZE);
+	             g_blemesh_stack, MYNEWT_VAL(BLE_MESH_ADV_STACK_SIZE));
 #endif
 
 	/* For BT5 controllers we can have fast advertising interval */

nimble/host/mesh/syscfg.yml

@@ -158,6 +158,13 @@ syscfg.defs:
             supported outgoing segment count (BT_MESH_TX_SEG_MAX).
         value: 6
 
+    BLE_MESH_ADV_STACK_SIZE:
+        description: >
+            Mesh advertiser thread stack size.
+            NOTE: This is an advanced setting and should not be changed unless
+            absolutely necessary
+        value: 768
+
     BLE_MESH_IVU_DIVIDER:
         description: >
             When the IV Update state enters Normal operation or IV Update