Add ability to check /etc/lsb-release for HWID

This is enabled by a compile-time option, only.

On certain boards HWID may not be available via normal means.
This functionality allows a compile-time choice to enable
behavior that will check /etc/lsb-release for an
HWID_OVERRIDE=<id> entry instead.

CQ-DEPEND=CL:186482
BUG=None
TEST=unit tests

Change-Id: I1279ea142a3b08dc4c50c527d8ee8f7b44c4a63f
Reviewed-on: https://chromium-review.googlesource.com/186483
Tested-by: Chris Masone <cmasone@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Commit-Queue: Chris Masone <cmasone@chromium.org>
diff --git a/SConstruct b/SConstruct
index 9513526..0fe2bf9 100644
--- a/SConstruct
+++ b/SConstruct
@@ -269,6 +269,7 @@
                    hardware.cc
                    http_common.cc
                    http_fetcher.cc
+                   hwid_override.cc
                    install_plan.cc
                    libcurl_http_fetcher.cc
                    marshal.glibmarshal.c
@@ -320,6 +321,7 @@
                             gpio_mock_udev_interface.cc
                             graph_utils_unittest.cc
                             http_fetcher_unittest.cc
+                            hwid_override_unittest.cc
                             metadata_unittest.cc
                             mock_http_fetcher.cc
                             mock_system_state.cc