Parse setting paths before parsing main config file

Parse setting paths before parsing main config file

Currently, paths either configure default values or append to a
list (When path_default is False, it's always a list based setting).

When paths are configuring default values, it makes more sense for
default values set in the mkosi.conf file to override path based
default values.

When appending to a list, (e.g. ExtraTrees=), it makes more sense
for the trees configured in the mkosi.conf to come after the tree
from the path (mkosi.extra).

Both these goals are achieved by parsing the path based values before
parsing the main mkosi.conf file.