clean up lwip

vendor/lwip/lwip/src/core/mem.c

@@ -179,6 +179,7 @@ struct mem {
  * how that space is calculated). */
 #ifndef LWIP_RAM_HEAP_POINTER
 /** the heap. we need one struct mem at the end and some room for alignment */
+ __attribute__ ((section(".data.$RAM4")))
 u8_t ram_heap[MEM_SIZE_ALIGNED + (2*SIZEOF_STRUCT_MEM) + MEM_ALIGNMENT];
 #define LWIP_RAM_HEAP_POINTER ram_heap
 #endif /* LWIP_RAM_HEAP_POINTER */