Move $(HOST_DIR)/usr/lib to $(HOST_DIR)/lib

This is a step towards eliminating $(HOST_DIR)/usr. It allows us to
convert all packages installing things into $(HOST_DIR)/usr/lib without
affecting the rest.

To allow compatibility with packages that still use $(HOST_DIR)/usr as
the prefix, create a symlink from usr/lib to ../lib.

Note that the symlink creation will break when $(HOST_DIR)/usr/lib
already exists as a directory, i.e. when rebuilding in an existing
output directory. This is necessary: if we don't break it now, the
following commits (which remove the usr part from various variables)
_will_ break it.

At the same time as creating this symlink, we also have to update the
check-host-rpath script to accept both $(HOST_DIR)/usr/lib and
$(HOST_DIR)/lib, because depending on how the package derives the
path, it may be different.

Since there are some dependency chains that involve $(STAGING_DIR),
$(STAGING_DIR) may in fact be created before $(HOST_DIR). Since
$(STAGING_DIR) is a subdirectory of $(HOST_DIR), it is possible that the
newly added rule for $(HOST_DIR) never triggers. To make sure that the
rule does trigger, add an order-only dependency from $(STAGING_DIR) to
$(HOST_DIR).

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>