I've reported the warnings to the repo owner: https://github.com/jwrdegoede/rtl8189ES_linux/issues/106 .. but don't hold breath.
You should be able to run the image from SD or USB cards. It still boots from eMMC, but the boot routine is modified to search for LE files on SD and then USB devices. In past testing I've found that some vendor u-boots struggle to init USB support and thus cannot find the device to boot from, but those seem to be the exception not the norm.