script: add ram and rom total size to json file

It's easier and safer to parse json file rather than extract
total size from stdout

Now the .json file has two keys:
{
    "symbols": {
       "children": [
           ...
	],
       "identifier": ":",
       "name": "root",
       "size": 2220
    },
    "total_size": 34272
}

Signed-off-by: Jingru Wang <jingru@synopsys.com>

scripts/footprint/size_report

@@ -12,6 +12,7 @@ import os
 import sys
 import re
 from pathlib import Path
+import json
 
 from distutils.version import LooseVersion
 
@@ -30,7 +31,7 @@ if LooseVersion(elftools.__version__) < LooseVersion('0.24'):
 from colorama import init, Fore
 
 from anytree import RenderTree, NodeMixin, findall_by_attr
-from anytree.exporter import JsonExporter
+from anytree.exporter import DictExporter
 
 
 # ELF section flags
@@ -634,10 +635,12 @@ def main():
         print_any_tree(root, symsize, args.depth)
 
         if args.json:
-            exporter = JsonExporter(indent=2, sort_keys=True)
-            data = exporter.export(root)
+            exporter = DictExporter()
+            data = dict()
+            data["symbols"] = exporter.export(root)
+            data["total_size"] = symsize
             with open(args.json, "w") as fp:
-                fp.write(data)
+                json.dump(data, fp, indent=4)
 
 
 if __name__ == "__main__":