This should replace the yaboot-bin package (which should be dropped
assuming this is found to be working well). This, instead of taking
the binary from Debian, compiles yaboot from source.
The tricky part here is e2fsprogs, or rather, libext2fs. Yaboot
requires a fairly old version of it, and being a strictly ppc32
program, needs that library compiled in a minimal form for ppc32.
In order to achieve that, I abuse the cross-compiler, using the
musl ppc32 cross-compiler as that's universally available on all
crosshosts, as well as all ppc64 (and ppc32 doesn't need it).
The main reason to do this over shipping the binary is that this
will actually work properly on some ppc64 machines, which require
the bundled addnote executable, which can now be compiled properly
for the native architecture.
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