Move $(HOST_DIR)/usr/$(GNU_TARGET_NAME) one level up.

This is a step towards eliminating $(HOST_DIR)/usr. It allows us to
convert all packages installing things into
$(HOST_DIR)/usr/$(GNU_TARGET_NAME) (i.e., binutils and gcc) without
affecting the rest.

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

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.

Effectively, the usr/ part is removed from $(STAGING_SUBDIR) (and
therefore from $(STAGING_DIR)), so update the definition of that
variable right away.

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>