That button serves to upload images to onboard NAND flash,if any.|
you connect a usb cable between the PC and the board's OTG port,fire up the image transfer software on the PC,point it to the correct image file and then press the Uboot button on the board and the image is flashed into NAND.
Since the BananaPi doesn't have onboard Flash,needless to say this function is redundant.
But wait...there is another function for the key.
It allows you to boot over USB...no not your pen drive plugged into the usb port,but something like network booting.
You plug in a usb cable between the OTG and your PC's USB port and use a special tool from Sunxi to load a specially prepared uBoot,your script.bin and the kernel,while pressing the uBoot key when powering up the BPi.This boots up the kernel for you and then it can be tailored to mount the rootfs.
As you can see...pretty tedious way of starting your Pi,but may come in handy if your sdcard slot's gone kaput or sth.