base-files: reduce debug prints

iopsys-brcm63xx-arm/base-files/lib/upgrade/iopsys-4-to-5-migrator.sh

@@ -788,7 +788,9 @@ nvram_old_convert() {
 	done
 
 	# Drop caches to ensure rereading just written data from flash.
-	echo 1 >/proc/sys/vm/drop_caches
+	sync
+	echo 4 >/proc/sys/vm/drop_caches
+	echo 3 >/proc/sys/vm/drop_caches
 	sleep 0.2
 
 	# Verify converted nvram.