Use olddefconfig for configuring kernel

This change updates the build script to use olddefconfig to use the old
.config file in the directory and use default values for unspecified
configuration options.

Fixes fabianishere#335

debian/rules

@@ -92,7 +92,7 @@ ${KERNEL_SRC}/.config:
 		${KERNEL_SRC}/debian.master/config/${DEB_BUILD_ARCH}/config.common.${DEB_BUILD_ARCH} \
 		${KERNEL_SRC}/debian.master/config/${DEB_BUILD_ARCH}/config.flavour.generic \
 		debian/config/config.pve
-	${MAKE} -C ${KERNEL_SRC} CC=${PVE_KERNEL_CC} oldconfig
+	${MAKE} -C ${KERNEL_SRC} CC=${PVE_KERNEL_CC} olddefconfig
 
 debian/${PVE_KERNEL_PKG}.%: debian/templates/pve-kernel.%.in
 	sed -e 's/@@KVNAME@@/${KVNAME}/g' < debian/templates/pve-kernel.$*.in > debian/${PVE_KERNEL_PKG}.$*