iperf: update to 3.21 and bundle in base image

[samtheruby]

Summary

• Bumps iperf 3.17.1 → 3.21 (esnet/iperf release 3.21), with the verified source-archive SHA256.
• Moves the package out of addons/addon-depends/network-tools-depends/ into packages/network/, since it's now a first-class base-image package (packages resolve by name, so the Network Tools addon still picks it up unchanged).
• Overrides makeinstall_target to ship only the iperf3 binary, avoiding libiperf.a, headers and man pages in the image.
• Adds iperf to the virtual/network metapackage so iperf3 is baked into every image at /usr/bin/iperf3 — no need to install the Network Tools addon.

Effect

• Base image: iperf3 always present at /usr/bin/iperf3.
• Network Tools addon: automatically inherits 3.21 (same package; it copies iperf3 and symlinks iperf → iperf3).

Testing

• Verified the 3.21 tarball URL + SHA256 (dd289b67…) and confirmed the binary builds at src/iperf3 (bin_PROGRAMS = iperf3).
• Full cross-compile from a cold tree could not be completed locally due to an unrelated kernel-headers patch failure blocking the target toolchain; the iperf package changes are independent of that.

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