scripts: Added better branch cksum checks

If we're fetching branches anyways, we might as well check that the
checksums match. This helps protect against infinite loops in B-tree
branches.

Also fixed an issue where we weren't xoring perturb state on finding an
explicit trunk.

Note this is equivalent to LFS_M_CKFETCHES in lfs.c.

---

This doesn't mean we always need LFS_M_CKFETCHES. Our dbg scripts just
need to be a little bit tougher because 1. running tests with -j creates
wildly corrupted and entangled littlefs images, and 2. Rbyd.fetch is
almost too forgiving in choosing the nearest trunk.

scripts/dbgrbyd.py

@@ -1003,7 +1003,8 @@ def main(disk, blocks=None, *,
                             if trunk and j_ + size > trunk:
                                 eoff_ = j_ + size
                                 eoff = eoff_
-                                cksum = cksum_
+                                cksum = cksum__ ^ (
+                                        0xfca42daf if perturb else 0)
                                 trunk_ = trunk__
                                 weight = weight_
                         trunk___ = 0