move $arch override to before it is used

closes #163 (closed)

• initial: arch=""
• _read_profile: packages="${profile}/packages.${arch}"
• _set_overrides: [[ -n "$arch" ]] || arch="$(uname -m)"

$arch is not defined in _read_profile if arch is not defined in profiledef.sh and packages is not updated after _set_overrides

[[ -n "$arch" ]] || arch="$(uname -m)" should be moved from _set_overrides to _read_profile.