remove g_multi and update the USB gadget configuration to be similar to Debian

@dorota.czaplejewicz could you test the usb gadget using this kernel? How do you test serial gadget until now? thanks! edit: nevermind. works.

this is targeted at byzantium only, so will be reverted for amber-phone

g_multi has a downstream hack and is legacy code so if we don't absolutely need it, we should remove it.