core/pkg-kconfig: don't enforce check for config file when not building

Currently, this is triggering the error message:
    make randconfig
    make source

Only limit the check to enforce a config file being set to when we are
actually building, like is done in the various kconfig-using packages.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>