[core] Allow calling UploadContext.collect_data multiple times

uf2tool/models/context.py

@@ -91,6 +91,9 @@ def get_offset(self, part: str, offs: int) -> Optional[int]:
             return None
         return start + offs
 
+    def rewind(self) -> None:
+        self.seq = 0
+
     def read_next(self, scheme: OTAScheme) -> Optional[Tuple[str, int, bytes]]:
         """
         Read next available data block for the specified OTA scheme.
@@ -152,6 +155,10 @@ def collect_data(self, scheme: OTAScheme) -> Optional[Dict[int, BytesIO]]:
         if not self.board_name:
             raise ValueError("This UF2 is not readable, since no board name is present")
 
+        # rewind to the beginning of the UF2 so collect_data can be called
+        # multiple times
+        self.rewind()
+
         out: Dict[int, BytesIO] = {}
         while True:
             ret = self.read_next(scheme)