Tweaked dbg script headers to match the mount info log

The main difference being rendering the weight with a single letter "w"
prefix:

  $ ./scripts/dbglfs.py disk -b4096
  littlefs v0.0 4096x256 0x{1,0}.8b w2.512, rev eb7f2a0d
  ...

This lets us add valuable weight info without too much noise.

Adopting this in the dbg scripts is nice for consistency.

scripts/dbgbtree.py

@@ -610,8 +610,11 @@ def main(disk, roots=None, *,
 
         # fetch the root
         btree = Rbyd.fetch(f, block_size, roots, trunk)
-        print('btree %s, rev %08x, weight %d, cksum %08x' % (
-            btree.addr(), btree.rev, btree.weight, btree.cksum))
+        print('btree %s w%d, rev %08x, cksum %08x' % (
+            btree.addr(),
+            btree.weight,
+            btree.rev,
+            btree.cksum))
 
         # look up a bid, while keeping track of the search path
         def btree_lookup(bid, *,