For reproducible builds the calculation of the filesystem UUID
should be persistent with each rebuild of the image. To achieve
this the UUID is calculated using the SOURCE_DATE_EPOCH from
the environment plus a char-number representation of the filesystem
label name as random seed. In kiwi every filesystem is created
with a label, thus only in case there is no SOURCE_DATE_EPOCH
available we continue to create the UUID as random data.
This Fixes#2761
8038ccfbef