pkg-rebar: use $(HOST_DIR) instead of $(HOST_DIR)/usr as prefix

pkg-rebar: use $(HOST_DIR) instead of $(HOST_DIR)/usr as prefix

Remove the redundant usr/ component of the HOST_DIR paths. Since a
previous commit added a symlink from $(HOST_DIR)/usr to $(HOST_DIR),
everything keeps on working.

$(PKG)_ERLANG_LIBDIR is problematic because it is used both for host
and staging/target. Therefore, the usr/ part is removed from it, and
added the the callers instead.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Frank Hunleth <fhunleth@troodon-software.com>
Reviewed-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>