Recursively merge extra_targets into targets

Recursively merge any target configs in extra_targets.json rather than completely replacing keys at the top level

tools/targets/__init__.py

@@ -22,7 +22,7 @@ import shutil
 import inspect
 import sys
 from copy import copy
-from collections import namedtuple
+from collections import namedtuple, Mapping
 from tools.targets.LPC import patch
 from tools.paths import TOOLS_BOOTLOADERS
 from tools.utils import json_file_to_dict
@@ -125,6 +125,23 @@ class Target(namedtuple("Target", "name json_data resolution_order resolution_or
     # Current/new location of the 'targets.json' file
     __targets_json_location = None
 
+    @staticmethod
+    def _merge_dict(dct, merge_dct):
+    """ Recursive dict merge. Inspired by `dict.update()` however instead of
+    updating only top-level keys, dict_merge recurses down into dicts nested
+    to an arbitrary depth, updating keys. 
+    The provided  ``merge_dct`` is merged into ``dct`` in place.
+    :param dct: dict onto which the merge is executed
+    :param merge_dct: dct merged into dct
+    :return: None
+    """
+    for k, v in merge_dct.iteritems():
+        if (k in dct and isinstance(dct[k], dict)
+                and isinstance(merge_dct[k], Mapping)):
+            Target._merge_dict(dct[k], merge_dct[k])
+        else:
+            dct[k] = merge_dct[k]
+
     @staticmethod
     @cached
     def get_json_target_data():
@@ -135,7 +152,7 @@ class Target(namedtuple("Target", "name json_data resolution_order resolution_or
         # If extra_targets.json exists in working directory load it over the top
         extra = os.path.join('.', 'extra_targets.json')
         if os.path.exists(extra):
-            targets.update(json_file_to_dict(extra))
+            Target._merge_dict(targets, json_file_to_dict(extra))
 
         return targets