Race condition with Raspberry Pi 3 root on NFS

["Aaron B." <aaron%zadzmo.org@localhost>]

My desired state is: kernel and bootstrap on the SD card, except with
the root filesystem on an NFS server.

I started with the official 9.3 aarch64 image, modified cmdline.txt to:

> root=mue0 console=fb

It tries, but fails. Seems the network interface is actually USB, which
doesn't attach before the kernel goes looking for the root filesystem:

[   5.6071214] boot device: ld0
[   5.6171226] unknown device major 0xffffffffffffffff
[   5.6171226] root device (default ld0a): mue0 at uhub2 port 1
[   7.0072744] mue0: vendor 0424 (0x424) product 7800 (0x7800), rev 2.10/3.00, addr 4 [   7.3151316] mue0: LAN7800 id 0x7800 rev 0x2
[   7.3329710] ukphy0 at mue0 phy 1: OUI 0x00800f, model 0x0013, rev. 2
[   7.3584591] ukphy0: 10baseT, 10baseT-FDX, 100baseTX, 100baseTX-FDX, 1000baseT, 1000baseT-FDX, auto 
[   7.3862316] mue0: Ethernet address b8:27:eb:f5:f3:a8 
[  19.2917853] mue0
[  20.4919454] dump device: mue0
[  23.5923589] file system (default generic): 
[  24.6524995] root on mue0 dumps on mue0
[  24.6562647] nfs_boot: trying DHCP/BOOTP
[  27.7029083] nfs_boot: DHCP next-server: 192.168.102.20

The 'mue0' at 19.29 seconds is me typing my preferred root device into
a prompt that I believe prints at 5.61 seconds, two seconds before the
network interface attaches.

The Pi 3 reaches multiuser after that, but I have to manually retype
the root FS each time it boots.

This looks less like a bug and more working as designed - but in an
weird edge case. Are there any magic workarounds to get multiuser
without human attention?

-- 
Aaron B. <aaron%zadzmo.org@localhost>