lib/constants.py: re-add IMAGE_TYPE_CR50_FIRMWARE

We need to keep the CR50 as an alias to the new value with a todo
to delete in the future.

BUG=b:173049030
TEST=./run_tests

Change-Id: I948d56cca4fad58c8ba65e806a5ad5fe7a67a776
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2582962
Tested-by: George Engelbrecht <engeg@google.com>
Reviewed-by: George Engelbrecht <engeg@google.com>
Auto-Submit: George Engelbrecht <engeg@google.com>
diff --git a/lib/constants.py b/lib/constants.py
index 36aeda1..c7eb490 100644
--- a/lib/constants.py
+++ b/lib/constants.py
@@ -824,6 +824,8 @@
 IMAGE_TYPE_ACCESSORY_RWSIG = 'accessory_rwsig'
 # GSC Firmware.
 IMAGE_TYPE_GSC_FIRMWARE = 'gsc_firmware'
+# TODO(b/173049030): Deprecate this alias after 2021-06.
+IMAGE_TYPE_CR50_FIRMWARE = IMAGE_TYPE_GSC_FIRMWARE
 
 IMAGE_TYPE_TO_NAME = {
     IMAGE_TYPE_BASE: BASE_IMAGE_BIN,