realtek: rename u-boot-env2 to board-name

realtek: rename u-boot-env2 to board-name

Some realtek boards have two u-boot-env partitions. However, in the
DGS-1210 series, the mtdblock2 partition is not a valid u-boot env
and simply contains the board/device name, followed by nulls.

00000000  44 47 53 2d 31 32 31 30  2d 32 38 2d 46 31 00 00 |DGS-1210-28-F1..|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00 |................|
*
00040000

00000000  44 47 53 2d 31 32 31 30  2d 35 32 2d 46 31 00 00 |DGS-1210-52-F1..|
00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00 |................|
*
00040000

The misleading u-boot-env2 name also confuses uboot-envtools.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>