bootperf: tolerate different occurrences of optional keyval

showbootdata aborts and prints nothing if the results dir contains
different occurrences of some keyval. For example
seconds_network_wifi_ready and seconds_network_ethernet_ready are likely
to have different occurrences, which is working as intended. Support
such data and prints a warning when this happens.

BUG=b:161488133
TEST=showbootdata displays boot statistics for the bug attachment

Change-Id: I2f314d731eaa141751aa2a2614f5c551459efa97
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/2784350
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Tested-by: Chinglin Yu <chinglinyu@chromium.org>
Commit-Queue: Chinglin Yu <chinglinyu@chromium.org>
2 files changed