detachables: Disable Network Boot Option

BUG=b:65595963
BRANCH=None
TEST=enabled Dev mode, reboot, select developer options &
     make sure Network Boot doesn't appear in menu.

Change-Id: I30953d037740259b06011ecb141d508db114b79e
Signed-off-by: Shelley Chen <shchen@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/664298
Reviewed-by: Julius Werner <jwerner@chromium.org>
diff --git a/firmware/lib/vboot_ui_menu.c b/firmware/lib/vboot_ui_menu.c
index 89865ee..8ca7a9f 100644
--- a/firmware/lib/vboot_ui_menu.c
+++ b/firmware/lib/vboot_ui_menu.c
@@ -642,6 +642,9 @@
 	    (flags & VBSD_BOOT_DEV_SWITCH_ON)) {
 		disabled_idx_mask |= 1 << VB_RECOVERY_TO_DEV;
 	}
+	/* Disable Network Boot Option */
+	if (current_menu == VB_MENU_DEV)
+		disabled_idx_mask |= 1 << VB_DEV_NETWORK;
 	return VBERROR_SUCCESS;
 }