bootperf: fix showbootdata arg parse for directories.

The positional argument for results directories should be parsed as
nargs="*" to return an array. This fixes showbootdata not running from
the bootperf results dir.

BUG=b:166744231, b:161488133
TEST=showbootdata <bootperf result>

Change-Id: I9f47bef9551fae876779428f45ea9b6009967663
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/2772415
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Tested-by: Chinglin Yu <chinglinyu@chromium.org>
Commit-Queue: Chinglin Yu <chinglinyu@chromium.org>
diff --git a/performance/bootperf-bin/showbootdata b/performance/bootperf-bin/showbootdata
index 25412bf..31acd97 100755
--- a/performance/bootperf-bin/showbootdata
+++ b/performance/bootperf-bin/showbootdata
@@ -102,7 +102,7 @@
 
   # Positional arguments (results_directory)
   argparser.add_argument('results_directory',
-                         help='bootperf results directory.', nargs='?')
+                         help='bootperf results directory.', nargs='*')
   return argparser