Merge pull request #9872 from KariHaapalehto/burst_nonblock_test_fix

Fix for NUCLEO_F207 UDPSOCKET_ECHOTEST_BURST_NONBLOCK fails.

Author: 0xc0170

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

@@ -6,7 +6,7 @@
     },
     "target_overrides": {
         "NUCLEO_F207ZG": {
-            "eth-rxbufnb": 1,
+            "eth-rxbufnb": 2,
             "eth-txbufnb": 4
         }
     }