automatically detect labels

[bastelfreak]

all labels are present in /srv/tftp/pxelinux.cfg/default

we've to define labels for the API in marmoset.conf + their callback methods, all labels present in default should work, without listing them in the marmoset.conf.

The current way is very error-prone because somebody who edits the default file will probably forget to also edit the marmoset.conf