ar71xx: image: fix TP-Link TL-WR710N v2/v2.1 BOARDNAME

All TP-Link TL-WR710N versions share the same machine code.

This has been working since the beginning as we don't use double-quotes
to protect spaces inside command line values. Thus, kernel interprets
'board=TL-WR710N v2' as 'board=TL-WR710N' and separate parameter 'v2'.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>