uARM targets now build in correct ARM_MICRO directory

This is for OS2 testing. Previously, targets that were configured with
the option "default_toolchain": "uARM" would still build to a
TOOLCHAIN_ARM_STD directory. This fixes these targets to build into
TOOLCHAIN_ARM_MICRO.

tools/build_api.py

@@ -121,6 +121,15 @@ def add_result_to_report(report, result):
     result_wrap = {0: result}
     report[target][toolchain][id_name].append(result_wrap)
 
+def get_toolchain_name(target, toolchain_name):
+    if toolchain_name == "ARM":
+        if CORE_ARCH[target.core] == 8:
+            return "ARMC6"
+        elif getattr(target, "default_toolchain", None) == "uARM":
+            return "uARM"
+
+    return toolchain_name
+
 def get_config(src_paths, target, toolchain_name=None, app_config=None):
     """Get the configuration object for a target-toolchain combination
 
@@ -316,8 +325,8 @@ def prepare_toolchain(src_paths, build_dir, target, toolchain_name,
         raise NotSupportedException(
             "Target {} is not supported by toolchain {}".format(
                 target.name, toolchain_name))
-    if (toolchain_name == "ARM" and CORE_ARCH[target.core] == 8):
-        toolchain_name = "ARMC6"
+
+    toolchain_name = get_toolchain_name(target, toolchain_name)
 
     try:
         cur_tc = TOOLCHAIN_CLASSES[toolchain_name]
@@ -942,6 +951,8 @@ def build_mbed_libs(target, toolchain_name, clean=False, macros=None,
     Return - True if target + toolchain built correctly, False if not supported
     """
 
+    toolchain_name = get_toolchain_name(target, toolchain_name)
+
     if report is not None:
         start = time()
         id_name = "MBED"