buildroot

core: introduce a generated kconfig snippet

Add the infrastructure for adding generated kconfig snippet in the
menuconfig.

For now, the kconfig snippet is generated empty, the recipe for filling
it in will be introduced in sub-sequent patches.

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>
Cc: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>