u-boot: clarify network settings submenu, disable override by default

Make it clear that these options override the board defconfig, and
don't enable it by default.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

boot/uboot/Config.in

@@ -92,10 +92,9 @@ config BR2_TARGET_UBOOT_FORMAT_LDR
 endchoice
 
 menuconfig BR2_TARGET_UBOOT_NETWORK
-	bool "Network Settings"
-	default y
+	bool "Custom Network Settings"
 	help
-	  Network settings for U-boot
+	  Custom network settings for U-boot
 
 if BR2_TARGET_UBOOT_NETWORK