If I add the meta-intel then meta-cedartrail layers within hob, I get the following parse error when I click OK:

ERROR: Unable to parse /home/paul/poky/meta-intel/meta-cedartrail/conf/layer.conf: Failure expanding variable BBPATH, expression was ${BBPATH}:/home/paul/poky/meta-intel which triggered exception Exception: variable BBPATH references itself!

From this and some experiments with "bitbake -p" after adding and reordering entries in bblayers.conf I was able to determine that this was because hob is reordering the list of layers internally; perhaps a dict is being used with the layer path as the key, although I haven't looked at the code. In any case, the order should at the very least not be different to the order in which the layers are shown within the layer dialog in hob; ideally it should remain the same as the order in which the user added them.