Enable ln in Busybox

[GregMefford]

This is the first step in enabling configfs and ultimately solving the following:

• Expose an RNDIS interface for Windows users nerves_system_rpi0#29
• Console doesn't work on Windows nerves_system_rpi0#45
• YAWI (Yet another Windows Issue) nerves_system_rpi0#46
• Configfs - DON'T MERGE nerves_runtime#58

We had high hopes that OTP21 would allow us to directly create symlinks in configfs paths from Elixir (using File.ln_s!), but it does not work. Writing the other files under configfs (using File.write!) does work in OTP 21, though.

[GregMefford]

Actually, never mind! I was just mis-using File.ln_s! in that you have to tell it the file name you want the symlink to have in the second argument (which is different than ln -s on the command-line, which infers the same file name as the source).

Hooray for OTP 21! 🎉