AU: Allow override of "appid" through the lsb-release file.

This will allow us to split Omaha configurations into OEM-specific files.

BUG=6145
TEST=unit tests, tested on device

Change-Id: I1f9b44d2a984988723ec81bf77594161c35460a2

Review URL: http://codereview.chromium.org/5022001
diff --git a/omaha_request_params.cc b/omaha_request_params.cc
index 19bcc6a..6884c83 100644
--- a/omaha_request_params.cc
+++ b/omaha_request_params.cc
@@ -47,7 +47,9 @@
       GetLsbValue("CHROMEOS_RELEASE_VERSION", "", NULL) : in_app_version;
   os_sp = app_version + "_" + GetMachineType();
   os_board = GetLsbValue("CHROMEOS_RELEASE_BOARD", "", NULL);
-  app_id = OmahaRequestParams::kAppId;
+  app_id = GetLsbValue("CHROMEOS_RELEASE_APPID",
+                       OmahaRequestParams::kAppId,
+                       NULL);
   app_lang = "en-US";
   app_track = GetLsbValue(
       kUpdateTrackKey,