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>

boot/grub/Config.in

@@ -58,6 +58,8 @@ config BR2_TARGET_GRUB_DISKLESS
 	help
 	  enable diskless support
 
+menu "network drivers"
+
 config BR2_TARGET_GRUB_3c595
 	bool "3Com595 driver"
 	help
@@ -163,4 +165,6 @@ config BR2_TARGET_GRUB_w89c840
 	help
 	  enable Winbond W89c840 driver
 
+endmenu
+
 endif # BR2_TARGET_GRUB