core/pkg-kconfig: do not always silence oldconfig

When debugging packages that use the pkg-kconfig infrastructure, it
is sometime interesting to see the exact command line that is being
executed when running oldconfig.

But currently, those command lines are prefixed with '@' which makes
them silent, always.

Instead, use $(Q) so they are silent by default, and verbose with V=1.

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