Some devices' downstream kernels may have different RNDIS/functionfs
drivers (ex. QCRNDIS) that require different values for some
configfs gadget attributes. Make it possible to customize with
deviceinfo file from device- package.
The idea is that each device- package can optionally specify a
bunch of `deviceinfo_usb_*` variables to customize the way it is
visible on USB bus. The ability to override
`deviceinfo_usb_rndis_function` is essential to make downstream
sdm660 based device to appear on USB bus.
While at it, make `idVendor`, `idProduct` and `serialnumber` also
customizable. So that we can make, for example, PineTab appear on
USB bus as PineTab, not as "Google Inc. Nexus 4 (fastboot)".
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