vpd: Fix dump_vpd_log failure on first run. dump_vpd_log aborts if the cache dir doesn't exist at the time temp files are created. Fixed by moving cache dir creation ahead of temp file creation. And a couple of minor fixes/improvments. BUG=chromium:379090, chromium:436056 TEST=manual test passed TEST=buildbot amd64-generic-full passed Change-Id: I1bab0695a473f3596c0714b0d9d1cc1b981b1fa6 Reviewed-on: https://chromium-review.googlesource.com/231334 Reviewed-by: Mattias Nissler <[email protected]> Tested-by: Thiemo Nagel <[email protected]> Reviewed-by: Hung-Te Lin <[email protected]> Commit-Queue: Thiemo Nagel <[email protected]>