ti_armv7_common: env: Add NFS loading support to default enviroment

NFS loading is similar to net loading except initial files are loaded
over NFS instead of TFTP, this removes the need for multiple different
protocol servers running on the host and allows the use of a single
network file system containing boot related files in their usual
in-filesystem directory. Add defaults for this boot style here.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>