touch_updater: Add log message for test case parsing for elants_i2c devices

By our customer's request, some log messages with specific pattern
should be added to /var/log/messages for test case parsing.
This patch adds log message with pattern "Product ID: <Product ID>".

Note: This patch might be duplicating another effort.

BUG=b:416642463
TEST=on jacuzzi device with elan i2c touchscreen, version check works.
After script finished, the specific log message can be found in
/var/log/messages.

Change-Id: Ia37aaefb548c058de97628fb020ecbee49bccda1
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/touch_updater/+/6577831
Reviewed-by: Paul Liang <paul.liang@emc.com.tw>
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
Reviewed-by: Henry Barnor <hbarnor@chromium.org>
Commit-Queue: Paul Liang <paul.liang@emc.com.tw>
Tested-by: Paul Liang <paul.liang@emc.com.tw>

scripts/chromeos-elan-touch-firmware-update-legacy.sh

diff --git a/scripts/chromeos-elan-touch-firmware-update-legacy.sh b/scripts/chromeos-elan-touch-firmware-update-legacy.sh
index 06f3932..cb46240 100755
--- a/scripts/chromeos-elan-touch-firmware-update-legacy.sh
+++ b/scripts/chromeos-elan-touch-firmware-update-legacy.sh
@@ -234,6 +234,9 @@
     die "Touch firmware updater: Product ID mismatch!"
   fi
 
+  # Log "Product ID: <Product ID>" to /var/log/messages for test case parsing.
+  log_msg "Product ID: ${active_product_id}"
+
   # Compare the touch device's FW version to the updater's.
   log_msg "Current Firmware: ${active_fw_version}"
   log_msg "Updater Firmware: ${fw_version}"