recovery: Separate blkid.tab for vold in recovery

vold does not have selinux permissions to read/write files in rootfs.
By default, libext2_blkid writes a cache file to /etc/blkid.tab. Set
environment variable BLKID_FILE just for use by the vold service,
changing the cache file location to /tmp/vold_blkid.tab which can then
take advantage of context vold_tmpfs.

Change-Id: I70d233d3d6f4e82bc7d781a04ef7707b733b4a1b