build_artifact: select correct signed image

xbuddy has a concept of "signed" image, but there are actually
a number of signed artifacts. The only one you'd be likely to
want to flash to USB would be the recovery image though.

This regex specifies that this should be a recovery image.

Example failing case:
xbuddy://remote/flapjack/latest-canary/signed ->
chromeos_12185.0.0_flapjack_factory_canary-channel_premp.bin,
chromeos_12185.0.0_flapjack_recovery_canary-channel_premp.bin
Currently fails due to two matches. Retreives the correct
image with this change.

BUG=chromium:959044
TEST=cros flash usb:// xbuddy://remote/flapjack/latest-canary/signed

Change-Id: If9c4d545246aea3f99196cbb2a544db9f064da90
Reviewed-on: https://chromium-review.googlesource.com/1600464
Commit-Ready: Nick Sanders <nsanders@chromium.org>
Tested-by: Nick Sanders <nsanders@chromium.org>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/build_artifact.py b/build_artifact.py
index 502b739..d9ccb16 100755
--- a/build_artifact.py
+++ b/build_artifact.py
@@ -47,7 +47,7 @@
 BASE_IMAGE_FILE = 'chromiumos_base_image.tar.xz'
 TEST_IMAGE_FILE = 'chromiumos_test_image.tar.xz'
 RECOVERY_IMAGE_FILE = 'recovery_image.tar.xz'
-SIGNED_IMAGE_FILE = 'chromeos_*mp*.bin'
+SIGNED_RECOVERY_IMAGE_FILE = 'chromeos_*recovery*mp*.bin'
 LIBIOTA_TEST_BINARIES_FILE = 'test_binaries.tar.gz'
 LIBIOTA_BOARD_UTILS_FILE = 'board_utils.tar.gz'
 QUICK_PROVISION_FILE = 'full_dev_part_*.bin.gz'
@@ -655,7 +655,8 @@
 _AddCrOSArtifact(artifact_info.RECOVERY_IMAGE, BundledArtifact, IMAGE_FILE,
                  optional_name=RECOVERY_IMAGE_FILE,
                  files_to_extract=[devserver_constants.RECOVERY_IMAGE_FILE])
-_AddCrOSArtifact(artifact_info.SIGNED_IMAGE, SignedArtifact, SIGNED_IMAGE_FILE)
+_AddCrOSArtifact(artifact_info.SIGNED_IMAGE, SignedArtifact,
+                 SIGNED_RECOVERY_IMAGE_FILE)
 _AddCrOSArtifact(artifact_info.DEV_IMAGE, BundledArtifact, IMAGE_FILE,
                  files_to_extract=[devserver_constants.IMAGE_FILE])
 _AddCrOSArtifact(artifact_info.TEST_IMAGE, BundledArtifact, IMAGE_FILE,