vpd: Add block_devmode_vpd helper script. Add helper script to update the block_devmode flag in VPD which uses cached data from dump_vpd_log to avoid unnecessary vpd calls. BUG=chrome-os-partner:50142 TEST=confirmed that VPD is set/cleared TEST=confirmed that the second of two identical calls is very fast CQ-DEPEND=CL:329114 Change-Id: I74d20f4f786705ffdc44b2ebf5dff3d2adede802 Reviewed-on: https://chromium-review.googlesource.com/329201 Commit-Ready: Thiemo Nagel <[email protected]> Tested-by: Thiemo Nagel <[email protected]> Reviewed-by: Hung-Te Lin <[email protected]>