pkg/kernel: use oldconfig instead of olddefconfig
pkg/kernel: use oldconfig instead of olddefconfig

See the added comment for motivation.

Update #501

google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller