cros_build_lib: Wait for LV to appear before mounting

Sometimes after running lvchange, the device takes a few seconds to
appear.  Check for the device and retry a couple of times as needed to
wait for it.  This is similar to the logic that was previously added to
wait for vgchange.

BUG=chromium:809570
TEST=cros tryjob edgar-paladin-tryjob

Change-Id: I77648232a7af36285253f5267ae2f90c4dfad063
Reviewed-on: https://chromium-review.googlesource.com/904731
Commit-Ready: Benjamin Gordon <bmgordon@chromium.org>
Tested-by: Benjamin Gordon <bmgordon@chromium.org>
Reviewed-by: Benjamin Gordon <bmgordon@chromium.org>
1 file changed