Add docs for building systems on macOS, and $TMPDIR

[schrockwell]

This is a follow-up to #876 and nerves-project/nerves_system_br#778 that adds documentation specifically about building custom systems on macOS, and the new $TMPDIR environment variable.

Should the docs recommend setting both $NERVES_DL_DIR and $NERVES_ARTIFACTS_DIR, or should we just set $XDG_DATA_HOME?

[fhunleth]

I don't have opinions here so long as it works. My plan would be to merge after new official Nerves systems are available with the nerves_system_br fix.

[schrockwell]

Yup, I think that all makes sense. Thanks.

[jjcarstens]

Why do the download and artifact directories need to be on the volume? I thought all that might be needed is TMPDIR since the build and mktemp would be using the copied temp files which would be available in case sensitive?

I don't think NERVES_DL_DIR needs to be recommended since that is just compressed tar and zip files. NERVES_ARTIFACTS_DIR is fine but we could also say XDG_DATA_HOME would be recommended since that would handle all the Nerves pieces

[schrockwell]

@jjcarstens You may be right about downloads, but for artifacts I did find conflicts such as

/Volumes/Nerves/artifacts/nerves_system_rpi3a_tailscale-portable-1.25.1/staging/usr/lib/xtables/libip6t_hl.so
/Volumes/Nerves/artifacts/nerves_system_rpi3a_tailscale-portable-1.25.1/staging/usr/lib/xtables/libip6t_HL.so

I'm happy to change it to XDG_DATA_HOME to make it simpler and potentially more future-proof.