Add DEPENDENCIES_HOST_PREREQ to the list of packages

That way packages included in that list like ccache will also be
regarded as a normal packages for targets like external-deps,
show-targets or legal-info

Signed-off-by: Alfredo Alvarez Fernandez <alfredo.alvarez_fernandez@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>