Added dbgbmap.py, tweaked tracebd.py to match

dbgbmap.py parses littlefs's mtree/btrees and displays that status of
every block in use:

  $ ./scripts/dbgbmap.py disk -B4096x256 -Z -H8 -W64
  bd 4096x256,   7.8% mdir,  10.2% btree,  78.1% data
  mmddbbddddddmmddddmmdd--bbbbddddddddddddddbbdddd--ddddddmmdddddd
  mmddddbbddbbddddddddddddddddbbddddbbddddddmmddbbdddddddddddddddd
  bbdddddddddddd--ddddddddddddddddbbddddmmmmddddddddddddmmmmdddddd
  ddddddddddbbdddddddddd--ddddddddddddddmmddddddddddddddddddddmmdd
  ddddddbbddddddddbb--ddddddddddddddddddddbb--mmmmddbbdddddddddddd
  ddddddddddddddddddddbbddbbdddddddddddddddddddddddddddddddddddddd
  dddddddddd--ddddbbddddddddmmbbdd--ddddddddddddddbbmmddddbbdddddd
  ddmmddddddddddmmddddddddmmddddbbbbdddddddd--ddbbddddddmmdd--ddbb

  (ok, it looks a bit better with colors)

dbgbmap.py matches the layout and has the same options as tracebd.py,
allowing the combination of both to provide valuable insight into what
exactly littlefs is doing.

This required a bit of tweaking of tracebd.py to get right, mostly
around conflicting order-based arguments. This also reworks the internal
Bmap class to be more resilient to out-of-window ops, and adds an
optional informative header.

scripts/dbgmtree.py

@@ -119,6 +119,9 @@ def crc32c(data, crc=0):
             crc = (crc >> 1) ^ ((crc & 1) * 0x82f63b78)
     return 0xffffffff ^ crc
 
+def popc(x):
+    return bin(x).count('1')
+
 def fromle32(data):
     return struct.unpack('<I', data[0:4].ljust(4, b'\0'))[0]
 
@@ -160,9 +163,6 @@ def frombtree(data):
     block, trunk, cksum = frombranch(data[d:])
     return w, block, trunk, cksum
 
-def popc(x):
-    return bin(x).count('1')
-
 def xxd(data, width=16):
     for i in range(0, len(data), width):
         yield '%-*s %-*s' % (
@@ -1646,8 +1646,8 @@ def main(disk, mroots=None, *,
                         # force next btree entry to be shown
                         prbyd = None
 
-        if args.get('error_on_corrupt') and corrupted:
-            sys.exit(2)
+    if args.get('error_on_corrupt') and corrupted:
+        sys.exit(2)
 
 
 if __name__ == "__main__":
@@ -1715,7 +1715,7 @@ if __name__ == "__main__":
     parser.add_argument(
         '-e', '--error-on-corrupt',
         action='store_true',
-        help="Error if B-tree is corrupt.")
+        help="Error if the filesystem is corrupt.")
     sys.exit(main(**{k: v
         for k, v in vars(parser.parse_intermixed_args()).items()
         if v is not None}))