Add loader.conf to the list of files that are MD.

loader.conf is also different between machines. On arm it's a minimal
one that's not quite compatible with the default one. On arm it's
minimal for speed, which is good, but there's also extra things in it
relative to the default on which break loading FDT which is bad. This
doesn't address that issue, but instead ensures the minimal one for
arm is used.

A similar issue for mips exists, but since we can have a beri variant
of /boot/loader and a uboot variant, I'm leaving that mess alone for
the moment.

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