realtek: build sane factory images for DGS-1210 models

realtek: build sane factory images for DGS-1210 models

During upload of firmware images the WebUI and CLI patch process
extracts a version information from the uploaded file and stores it
onto the jffs2 partition. To be precise it is written into the
flash.txt or flash2.txt files depending on the selected target image.
This data is not used anywhere else. The current OpenWrt factory
image misses this label. Therefore version information shows only
garbage. Fix this.

Before:
DGS-1210-20> show firmware information
IMAGE ONE:
Version      : xfo/QE~WQD"A\Scxq...
Size         : 5505185 Bytes

After:
DGS-1210-20> show firmware information
IMAGE ONE:
Version      : OpenWrt
Size         : 5505200 Bytes

Tested-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
(cherry picked from commit fae3ac3560459320a88be86b31c572c4bca42645)