Fix can't link XDEV using repos as pkgset_sources
    
    Trying to compose from external classic repositories return an error trying the hardling from
    a yum cache directory located in /tmp to the target directory in another filesystem.
    This commit fixes this using the 'link' method form linker module which handle the link_type
    configuration parameter instead of the hardcoded method 'hardlink'.
    
    Change-Id: Ib79cfbd72f9def6462fddb2ae368730c55f257cd
    Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>