devserver: Check whether devserver can run before rootfs update.

BUG=chromium:718532
TEST=Test locally: provision a DUT with old version of ChromeOS.

Change-Id: I3a7833ab2f81246015f8e228fd6de16509b93fa3
Reviewed-on: https://chromium-review.googlesource.com/500048
Commit-Ready: Xixuan Wu <[email protected]>
Tested-by: Xixuan Wu <[email protected]>
Reviewed-by: Allen Li <[email protected]>
2 files changed