build_image: set keyset field in lsb-release

This tells dev built images to include CHROMEOS_RELEASE_KEYSET=devkeys.

BUG=chromium:443736
TEST=precq passes
CQ-DEPEND=CL:1217266, CL:*676258

Change-Id: Iadf178acccddeafb86e25954d8f4583caccaee15
Reviewed-on: https://chromium-review.googlesource.com/1217267
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Jason Clinton <jclinton@chromium.org>
diff --git a/build_library/base_image_util.sh b/build_library/base_image_util.sh
index 2a957a0..c326148 100755
--- a/build_library/base_image_util.sh
+++ b/build_library/base_image_util.sh
@@ -303,6 +303,7 @@
     --board="${BOARD}" \
     "${model_flags[@]}" \
     ${builder_path} \
+    --keyset="devkeys" \
     --version_string="${CHROMEOS_VERSION_STRING}" \
     --auserver="${CHROMEOS_VERSION_AUSERVER}" \
     --devserver="${CHROMEOS_VERSION_DEVSERVER}" \