core: introduce an intermediate rule before the configurators

core: introduce an intermediate rule before the configurators

Currently, all configurators depend on generating the out-of-tree
Makefile wrapper.

In an upcoming patch, we'll need to also generate a kconfig fragment,
so it will have to kick in before we run the configurators.

Introduce a new intermediate "prepare-kconfig" rule, so we can
commonalise the dependencies of the configurators. Move the dependency
on the Makefile wrapper to that new intermediate rule.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <jacmet@uclibc.org>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
[Thomas: mark prepare-kconfig as a phony target.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>