check-host-rpath: no longer allow $(HOST_DIR)/usr

Now all packages have been updated to install things in $(HOST_DIR)/lib
instead of $(HOST_DIR)/usr/lib, there should no longer be any reason
to have $(HOST_DIR)/usr/lib in the RPATH, so we don't allow it any more.

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