MIPS: generic: Enable Root FS on NFS in generic_defconfig

The generic_defconfig is used for platforms like SEAD3 which do not
usually have fixed storage available, therefore NFS is the preferred
location of the RFS.
When the upstream kernel defconfig is built & tested on platforms such
as SEAD3 this leads to essentially false failures when the RFS fails to
mount.

There is little harm in having this feature enabled by default, so
enable it in the defconfig. Kernel autoconfiguration & DHCP must also be
selected to allow RFS on NFS.

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15853/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>