buildroot

pkg-python: 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.

Build-tested with a bunch of python packages.

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