tailscale: improve packaging

Maintainer: @BKPepe (?)
Environment: OpenWrt 21.02.3, r16554-1d4dea6d4f

Description:

The tailscale and tailscaled binaries are quite large, even when squashfs'd:

The tailscale documentation mentions that, exactly to address this, tailscaled can be optionally built including the cli functionality, so that it can be symlinked busybox-style to support both the tailscaled and tailscale cli roles:

go build -o tailscale.combined -tags ts_include_cli ./cmd/tailscaled
ln -s tailscale.combined tailscale
ln -s tailscale.combined tailscaled

As currently tailscale depends on tailscaled, we could maybe move from the current two packages

• tailscaled
• tailscale (depends: tailscaled)

to two packages and a virtual package

• tailscaled
• tailscale-combined (provides: tailscale, tailscaled)

where tailscale-combined is the tailscaled command built using the ts_include_ci tag.

Quickly building the packages locally and compressing with upx (as a stand-in for squashfs) seems to indicate that this approach would save ~1.3MB in a firmware image that includes tailscale (e.g. such as those built via attended-sysupgrades).

        File size         Ratio      Format      Name
   --------------------   ------   -----------   -----------
   9961472 ->   2308728   23.18%   linux/mips    tailscale
  26083328 ->   4790500   18.37%   linux/mips    tailscale.combined
  17235968 ->   3765896   21.85%   linux/mips    tailscaled

Start by locating the OpenWrt package definitions for tailscaled and tailscale, then review the tailscale documentation on small builds and the ./cmd/tailscaled entry point. Compare separate builds with a ts_include_cli combined build, and verify that the resulting packages provide the requested tailscale and tailscaled commands while reducing installed size.