Enable Environment Variables in Configuration File

This code change enables the use of Linux Environment
variables in the configuration file. This is very useful
for paths to avoid having to hard-code paths.

Added error checking on the environment variables.

For proper use, it will also require a similar patch to
the mixer-tools project.

Signed-off-by: Mark Horn <mark.d.horn@intel.com>

bundle-chroot-builder.py

@@ -25,6 +25,7 @@ import argparse
 import configparser
 import os
 import os.path
+import io
 import platform
 import re
 import shutil
@@ -54,9 +55,20 @@ def get_config(args):
     if args.config:
         buildconf = args.config
 
-    config = configparser.ConfigParser()
     print("Reading from %s" % buildconf)
-    config.read(buildconf)
+    cfg_txt = ""
+    # Check that the environment variables in the config file are valid
+    pattern = re.compile("\$\{?(\w+)\}?")
+    for i, line in enumerate(open(buildconf, 'r')):
+        for match in re.finditer(pattern, line):
+            if not match.group(1) in os.environ:
+                print("ERROR:\nbuilder.conf contains an undefined environment variable: %s on line %s\n"
+                        % (i+1, match.group(1)))
+                exit(1)
+        cfg_txt += os.path.expandvars(line)
+
+    config = configparser.ConfigParser()
+    config.readfp(io.StringIO(cfg_txt))
     return config