ar71xx-generic: Remove extra dash between v2 and OpenMesh device name (#781)

The returned name for OpenMesh devices with a an extra vX when calling

    lua -e 'print(require("platform_info").get_image_name())'

doesn't contain a dash between the vX and the device name. Thus the image
should also not contain a dash.

Signed-off-by: Sven Eckelmann <sven.eckelmann@open-mesh.com>