realtek: make DGS-1210 u-boot-env partition writeable

realtek: make DGS-1210 u-boot-env partition writeable

We are close to provide enduser friendly OpenWrt images for DGS-1210
switches that do not need serial console. Nevertheless a small bit is
missing. We cannot switch back to the vendor partition or initiate a
download of a vendor firmware image. To issue this from inside OpenWrt
we need write access to U-Boot environment.

Case 1: Switch back to secondary (vendor) image
> fw_setenv bootcmd run addargs\; bootm 0xb4e80000
> fw_setenv image /dev/mtdblock7
> reboot

Case 2: Issue D-Link Network Assistant based download on next reboot.
This is a combination of some vendor specific protocol (DDP) and a
TFTP download afterwards.
> fw_setenv bootstop on
> reboot

Allow these commands by opening up u-boot-env for write access.
Tested on DGS-1210-20.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>