@prudy, thank you very much. (Now I don't have to wait rpi0 for the gadget test :-)
So, after lots of failed attempts, now my rpiA can "rpiboot" into gadget mode.
(i.e. Booting without SD card into gadget mode.)
My (experimental) setup's boot sequence:-
- rpiboot downloads usbbootcode.bin.
- rpiboot downloads buildroot.elf + fat_image+fit_image.
- buildroot.elf starts u-boot.bin (in the fat_image.)
- u-boot.bin starts linux_kernel(+device_tree+ramdisk,in the fit_image.)
it is a great learning process for me. But, I understand these U-Boot things are not requred with the upcoming boot files (https://github.com/raspberrypi/tools/issues/44