Add HWID information for ARM via crossystem

Added HWID query via crossystem if not available via ACPI. This populates
the hwclass field for crash reports.

BUG=None
TEST=Manually confirmed ARM platform crash reports contains correct hwclass
and manually confirmed ZGB report contains previously reported hwclas

Change-Id: I1216ed13c0b04d829313103d3bb736f5227a772b
Reviewed-on: http://gerrit.chromium.org/gerrit/7982
Reviewed-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Micah Catlin <micahc@chromium.org>
Commit-Ready: Katie Roberts-Hoffman <katierh@chromium.org>
Tested-by: Katie Roberts-Hoffman <katierh@chromium.org>

crash_reporter/crash_sender

@@ -196,6 +196,8 @@ get_board() {
 get_hardware_class() {
   if [ -r "${HWCLASS_PATH}" ]; then
     cat "${HWCLASS_PATH}"
+  elif crossystem hwid > /dev/null 2>&1; then
+    echo "$(crossystem hwid)"
   else
     echo "undefined"
   fi