grub: enclose network driver options in a sub-menu

In preparation to a change to the configuration options of the
supported filesystem in Grub, enclose the network driver options in a
sub-menu.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>