toolchain-wrapper: remove remaining references to HOST_DIR/usr

Commit 14151d77 that eliminated
$(HOST_DIR)/usr seriously missed the toolchain-wrapper - only a single
reference was updated, the other three were missed. Commit
015d68c8 removed one more. This commit
finally removes the two remaining ones.

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