scripts: Tweaked bd.read to behave like an actual bd_read callback

This better matches what you would expect from a function called
bd.read, at least in the context of littlefs, while also decreasing the
state (seek) we have to worry about.

Note that bd.readblock already behaved mostly like this, and is
preferred by every class except for Bptr.

scripts/dbgrbyd.py

@@ -309,13 +309,10 @@ class Bd:
     def repr(self):
         return 'bd %sx%s' % (self.block_size, self.block_count)
 
-    def read(self, size=-1):
+    def read(self, block, off, size):
+        self.f.seek(block*self.block_size + off)
         return self.f.read(size)
 
-    def seek(self, block, off=0, whence=0):
-        pos = self.f.seek(block*self.block_size + off, whence)
-        return pos // self.block_size, pos % self.block_size
-
     def readblock(self, block):
         self.f.seek(block*self.block_size)
         return self.f.read(self.block_size)