Globally replace $(HOST_DIR)/usr with $(HOST_DIR)

Since things are no longer installed in $(HOST_DIR)/usr, the callers
should also not refer to it.

This is a mechanical change with
git grep -l '\$(HOST_DIR)/usr' | xargs sed -i 's%\$(HOST_DIR)/usr%$(HOST_DIR)%g'

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