Makefile: move mkdir rule to after HOST_DIR is defined

Makefile: move mkdir rule to after HOST_DIR is defined

HOST_DIR is defined twice: once to its default value before .config is
included, and once more to BR2_HOST_DIR after .config is included.
However, the rule that defines the mkdir for HOST_DIR comes between
these two, so it will always use the default definition. Therefore,
if a non-default BR2_HOST_DIR is used, there will be no rule to create
that directory, while the dirs target depends on it.

This happens to work at the moment, because in the dirs target,
$(STAGING_DIR) comes before $(HOST_DIR), so $(HOST_DIR) will be created
implicitly. However, this will fail in top-level parallel builds where
both will be created in parallel.

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