buildroot

core/pkg-kconfig: Factorize make commands

Use a variable to factorize the calls to make, as we use the same set
of options multiple times.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>