Don't consume 36 kB just for Ethernet buffers.

Kintis EMAC is consuming 16 rinbuffers for input, and 8 buffers for output.
This is over-use because input packets are immediately allocated from
heap when passed to LwIP. Therefore the number can be creatly reduced.

features/netsocket/emac-drivers/TARGET_Freescale_EMAC/mbed_lib.json

@@ -1,7 +1,7 @@
 {
     "name": "kinetis-emac",
     "config": {
-        "rx-ring-len": 16,
+        "rx-ring-len": 2,
-        "tx-ring-len": 8
+        "tx-ring-len": 1
     }
 }