Merge pull request #156 from ARMmbed/use_std_lib

Use c_lib std by default

Author: teetak01

mbed_app.json

@@ -10,6 +10,7 @@
     "target_overrides": {
         "*": {
             "target.features_add"                       : ["BOOTLOADER", "STORAGE"],
+            "target.c_lib"                              : "std",
             "platform.stdio-baud-rate"                  : 115200,
             "platform.stdio-convert-newlines"           : true,
             "platform.stdio-buffered-serial"            : true,