ARMV7: OMAP3: Fix build failure for igep0030 machine

Seems that if CONFIG_CMD_NET is undefined we should also
undefine CONFIG_CMD_NFS, otherwise build fails with various
undefined reference like:

  net/libnet.o: In function `rpc_req':
  u-boot/net/nfs.c:193: undefined reference to `NetEthHdrSize'
  u-boot/net/nfs.c:202: undefined reference to `NetSendUDPPacket'
  u-boot/net/nfs.c:203: undefined reference to `NetTxPacket'
  u-boot/net/nfs.c:203: undefined reference to `NetServerEther'

This patch adds the undef CONFIG_CMD_NFS in configuration file.

Signed-off-by: Enric Balletbo i Serra <eballetbo@iseebcn.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>