Use 1k value for uart rxbuf size

Previously esp-driver specified internally the buffer to be 1024.
Now this is defined inside Mbed OS and the default value there is
256. This is too small in most usecases.

Author: teetak01

configs/wifi_esp8266_v4.json

@@ -24,6 +24,7 @@
     "macros": ["MBEDTLS_USER_CONFIG_FILE=\"mbedtls_mbed_client_config.h\""],
     "target_overrides": {
         "*": {
+            "drivers.uart-serial-rxbuf-size": 1024,
             "target.features_add": ["COMMON_PAL"],
             "platform.stdio-baud-rate": 115200,
             "platform.stdio-convert-newlines": true,