tools: Support default_lib and c_lib

For backwards compatibility reasons, since the latest Mbed OS tools must
be able to build any previous online-compiler-supported version of Mbed
OS, allow targets to use `default_lib` or `c_lib`. This should enable
the tools to work with Mbed OS 5 style targets.json.

Author: Patater

tools/toolchains/mbed_toolchain.py

@@ -1097,11 +1097,12 @@ def check_c_lib_supported(self, target, toolchain):
         target.c_lib is modified to have the lowercased string of its original string.
         This is done to be case insensitive when validating.
         """
-        if  hasattr(target, "default_lib"):
-            raise NotSupportedException(
-                   "target.default_lib is no longer supported, please use target.c_lib for C library selection."
-                )
-        if  hasattr(target, "c_lib"):
+        if hasattr(target, "default_lib"):
+            # Use default_lib as the c_lib attribute. This allows backwards
+            # compatibility with older target definitions, allowing either
+            # default_lib or c_lib to specify the C library to use.
+            target.c_lib = target.default_lib.lower()
+        if hasattr(target, "c_lib"):
             target.c_lib = target.c_lib.lower()
             if (
                 hasattr(target, "supported_c_libs") == False