Makefile: create symlink to non-default HOST_DIR

Makefile: create symlink to non-default HOST_DIR

If BR2_HOST_DIR is not the default, it can be difficult to find the
host directory (i.e., HOST_DIR always has to be passed explicitly in
addition to the output directory). For example, the Eclipse plugin
assumes that HOST_DIR=BASE_DIR/host.

Create a symlink from $(BASE_DIR)/host to $(HOST_DIR) if it is not the
default. Also remove it in the clean target.

When BR2_HOST_DIR is the default, HOST_DIR_SYMLINK will be empty so
there will be no additional dependency to dirs and nothing to remove
in clean.

Fixes https://bugs.busybox.net/show_bug.cgi?id=10151

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