Eliminate $(HOST_DIR)/usr

Eliminate $(HOST_DIR)/usr

We currently use $(HOST_DIR)/usr as the prefix for host packages. That
has a few disadvantages:

- There are some things installed in $(HOST_DIR)/etc and
  $(HOST_DIR)/sbin, which is inconsistent.

- To pack a buildroot-built toolchain into a tarball for use as an
  external toolchain, you have to pack output/host/usr instead of the
  more obvious output/host.

- Because of the above, the internal toolchain wrapper breaks which
  forces us to work around it (call the actual toolchain executable
  directly). This is OK for us, but when used in another build system,
  that's a problem.

- Paths are four characters longer.

To allow us to gradually eliminate $(HOST_DIR)/usr while building
packages, replace it with a symlink to .

The symlinks from $(HOST_DIR)/usr/$(GNU_TARGET_NAME) and
$(HOST_DIR)/usr/lib that were added previously are removed again.

Note that the symlink creation will break when $(HOST_DIR)/usr
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 have to update the
external toolchain wrapper and the external toolchain symlinks to go
one directory less up. Indeed, $(HOST_DIR) is one level less up than
it was before.

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>