generic packages: 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.

This is a mechanical change with
git grep -l '\$(HOST_DIR)/usr' | xargs sed -i 's%\(prefix\|PREFIX\)=\("\?\)\$(HOST_DIR)/usr%\1=\2$(HOST_DIR)%g'

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>