core: introduce intermediate BASE_TARGET_DIR variable

core: introduce intermediate BASE_TARGET_DIR variable

This new BASE_TARGET_DIR variable is set in stone to point to the real
location where packages will be installed. Its name is modelled after
its definition: it is located in $(BASE_DIR), and it is named 'target/',
hence BASE_TARGET_DIR.

The already-existing TARGET_DIR variable now simply points to the same
location, except that it is recursively expanded, so that we can later
change it depending on the context.

All locations that really need to reference the existing target/
directory, are changed to use BASE_TARGET_DIR; surprinsigly enough, they
all seem to be located in the main Makefile. :-) The rest is left with
using good-old TARGET_DIR.

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