This is not necessary anymore on newer versions of OpenWrt. When you flash a sysupgrade firmware on newer versions of OpenWrt via the web interface it will provide you full access to the flash.
If you've used OpenWrt on UniFi AC Lite/LRs, you may know that the overlayfs only provides 3.6 MB of space. If you think about it, that's kinda weird, because all the hardware docs say that the access point has 16 MB of flash, so where did all the other 12.4 MBs go? Surely they're not all being taken up by the OS.