micropython

Remove -r from ln
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Quote from Unix Stack Exchange:

> Beware that -r is a GNUism, i.e. non POSIX so won't work [...] as the
> standard OS X ln command is BSD based.

https://unix.stackexchange.com/questions/125132/ln-s-with-a-path-relative-to-pwd#comment455718_263512

Fix tr and dd for macOS
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See Unix Stack Exchange question "Filling file with 0xFF gives C3BF in OSX" at

    https://superuser.com/a/1349497/150212

and

https://lists.freebsd.org/pipermail/freebsd-questions/2007-October/160961.html