Kinetis EMAC: Make number of buffers configurable

16 RX buffers and 8 TX buffers is probably excessive. Nanostack
version of driver successfully used 4+4, and data pump should be
broadly equivalent.

This means that switching K64F devices from Nanostack to EMAC increases
base heap usage by 18K - observed in Nanostack border router builds.

Add a config option to make it possible to lower the number of buffers.
Defer consideration of lowering the default to later.

features/netsocket/emac-drivers/TARGET_Freescale_EMAC/kinetis_emac_config.h

@@ -32,8 +32,8 @@
 
 #include "fsl_enet.h"
 
-#define ENET_RX_RING_LEN              (16)
-#define ENET_TX_RING_LEN              (8)
+#define ENET_RX_RING_LEN              MBED_CONF_KINETIS_EMAC_RX_RING_LEN
+#define ENET_TX_RING_LEN              MBED_CONF_KINETIS_EMAC_TX_RING_LEN
 
 #define ENET_ETH_MAX_FLEN             (1522) // recommended size for a VLAN frame
 

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

@@ -0,0 +1,7 @@
+{
+    "name": "kinetis-emac",
+    "config": {
+        "rx-ring-len": 16,
+        "tx-ring-len": 8
+    }
+}