build-sys: disable jobserver for rootfs target
build-sys: disable jobserver for rootfs target

2588e3dff5d broke the distcheck target. Fix it by serializing the rootfs
(re)creation prior to running the testsuite.

lucasdemarchi/kmod,lpereira/kmod,pmmccorm/kmod,Distrotech/kmod,lucasdemarchi/kmod,pmmccorm/kmod,Distrotech/kmod,lucasdemarchi/kmod,lpereira/kmod,lpereira/kmod,pmmccorm/kmod,Distrotech/kmod