selftest/buildhistory: Improve test to remove sources of error

selftest/buildhistory: Improve test to remove sources of error

When we add buildhistory, we expect do_rootfs to rerun, but depending
on IMAGE_FSTYPES, the number of tasks which would execute after
do_rootfs varies (e.g. live would add do_bootimg and we recently
added do_image).

Therefore limit the test to -c rootfs and then we're clear that only
one task should re-run.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>