PiZW usb hub and devices not handled by SG udev rules

[jbrzusto]

The problem is our custom rules are loaded from the root filesystem, which is only mounted
after the PiZW has enumerated the host controller (so we can't map the correct portnums file) and its
attached device (so the device gets enumerated before a portnums mapping has been established, and
before we even have a rule for the device).

Notes/TODO:

• investigate how the USB host controller and device enumeration differ between PiZW and Pi3; is this
  a kernel issue, or something to do with initramfs, or a change between debian versions?
• oddly, the FCD still gets handled, albeit with incorrect port number, so there's probably a stale version of
  SG udev rules (one without the rtlsdr rules, e.g.) in initramfs.
• copy SG udev rules etc. to initramfs so that it doesn't matter whether enum happens before or after
  mount of root fs? Alternatively, check whether we can re-order root FS mounting and USB enumeration
  via systemd wrangling.

[jbrzusto]

The only purpose of the initramfs is to support the Liwixi mount of root fs as a loopback ext4 image inside a VFAT file.
Perhaps we can radically slim down the initramfs?