Use PARTLABEL instead of LABEL

Using a label on the partition instead of on the file system has the
advantage of decoupling the label information from the file system which
will persist even to formatting the file system for example. Using
PARTLABEL is in general a more robust approach.

This patch also replaces `e2label` by `lsblk` as the former does not
require root permissions and also produces json output making it more
reliable to parse.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>