Switch to toybox du(1).

Test: treehugger
Change-Id: Id66705f35e82ed25af6dbd5a131ce90ceba60912
diff --git a/ui/build/paths/config.go b/ui/build/paths/config.go
index 604bf90..ec5c028 100644
--- a/ui/build/paths/config.go
+++ b/ui/build/paths/config.go
@@ -81,7 +81,6 @@
 	"date":      Allowed,
 	"dd":        Allowed,
 	"diff":      Allowed,
-	"du":        Allowed,
 	"echo":      Allowed,
 	"egrep":     Allowed,
 	"expr":      Allowed,
@@ -158,6 +157,7 @@
 	"cp":       Toybox,
 	"cut":      Toybox,
 	"dirname":  Toybox,
+	"du":       Toybox,
 	"env":      Toybox,
 	"head":     Toybox,
 	"id":       Toybox,