nusb/serial: fix nusb/serial hang with ftdi

have to create process, not a thread in ftdi code so main
process can exit after posting fs service.
zynq: fix kernel configuration so it builds
5l: remove -f flag and add -F flag to disable VFP code generation

almost nobody uses FPA anymore, so make VFP the default but
provide -F flag to disable it.
zynq: set mainmem->maxsize so kernel malloc() wont fail after 4MB
zynq: active.machs = 1