let blkid use /dev/null as cache file

in mer SDK to generate btrfs images blkid works more reliable with no
cache file.

Signed-off-by: Reto Zingg <reto.zingg@jollamobile.com>
Merge pull request #2 from d0b3rm4n/fix-blkid-issue-in-sdk

let blkid use /dev/null as cache file