Makefile: move definition of TARGET_DIR inside .config condition

In a follow-up commit introducing per-package directory support, we
will need to define TARGET_DIR in a different way depending on the
value of a Config.in option. To make this possible, the definition of
TARGET_DIR should be moved inside the BR2_HAVE_DOT_CONFIG condition.

We have verified that $(TARGET_DIR) is only used within the
BR2_HAVE_DOT_CONFIG condition. Outside of this condition, such as in
the "clean" target, $(BASE_TARGET_DIR) is used.

Suggested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>