bcmxcp_usb.c does not use nut_usb_addvars()

[jimklimov]

...and so lacks support for common USB-matching options like device, bus, serial.

A fix similar to that in #1763 was attempted but proved hard, at least in the way of moving upsdrv_makevartable() from common bcmxcp.c file to bcmxcp_ser.c and bcmxcp_usb.c, because this driver implements a fair amount of USB support privately and shares many symbols with libusbX.c files. This further complicates build/linking of nut-scanner as well.

Code from that attempt is parked at https://github.com/jimklimov/nut/tree/issue-1754-bcmxcp (jimklimov@9be75da)

No plan to address this soon, so keeping for posterity if someone does need to address it later and finds what I missed in the quick shot.