sunxi: Make ROOT_NFS usable out of the box on sun4i and sun7i

Tweak the defconfigs to enable relevant options. And also set
SoC-dependent LOCALVERSION identifiers, so that sun4i and sun7i
modules do not clash if installed on the same rootfs (which may
otherwise naturally happen in the case if multiple devices are
sharing the same NFS root).

Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>