AM3517:Build FIX: undef CONFIG_CMD_NFS support

Without CONFIG_CMD_NET support CONFIG_CMD_NFS leads
to linking error, so disable CONFIG_CMD_NFS option.

Signed-off-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>