kernel: Append /dev/{u,}random to initrd instead of dracut caps

[cgwalters]

Rather than giving dracut cap_mknod which won't work in
unprivileged scenarios, append a tiny static pre-generated CPIO
blob with /dev/random and /dev/urandom to the output of
dracut.

This is a hack until dracut does this itself. But the problem
is patches to dracut will take eleven billion years to ship
in RHCOS.

Closes: #1950

[cgwalters]

One surprising thing is lsinitrd doesn't do what the kernel does and follow the chain, but I did test this successfully with RHCOS 4.3.

[cgwalters]

Updated 🆕

[ashcrow]

I don't fully grok the reasoning but the code itself looks good and the cpio archive was what it said it was :-)

[jlebon]

/lgtm

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[cgwalters]

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