Support network boot via USB Ethernet adapters
Recent generations of notebooks quite often no longer provide Ethernet NICs, so booting via USB Ethernet adapters is often the only option there.
Without the net/usb kernel drivers in the initramfs, it's not possible to boot via PXE/network boot using a USB Ethernet adapter.
By not excluding the "usb" directory from auto_add_modules()'s "net" handling, we include the necessary net/usb drivers.
Closes: #980788