pkg/build: copy kernel.full to the objdir after a build
pkg/build: copy kernel.full to the objdir after a build

This helps ensure that we can create coverage reports on FreeBSD.

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