buildroot

Makefile: remove $(HOST_DIR)/usr from BR_PATH

Now $(HOST_DIR)/usr is a symlink to $(HOST_DIR), it makes no sense to
still have it in BR_PATH.

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>