[malloc] Increase heap size to 512kB

The maximum TCP throughput is fundamentally limited by the amount of
available receive buffer space.  Increase the heap size from 128kB to
512kB to allow the use of larger TCP windows.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/malloc.c

 /**
  * Heap size
  *
- * Currently fixed at 128kB.
+ * Currently fixed at 512kB.
  */
-#define HEAP_SIZE ( 128 * 1024 )
+#define HEAP_SIZE ( 512 * 1024 )
 
 /** The heap itself */
 static char heap[HEAP_SIZE] __attribute__ (( aligned ( __alignof__(void *) )));