build_dist.sh: keep --extra-small making a usable build, add --min

Historically, and until recently, --extra-small produced a usable build.

When I recently made osrouter be modular in 39e35379d4 (which is
useful in, say, tsnet builds) after also making netstack modular, that
meant --min now lacked both netstack support for routing and system
support for routing, making no way to get packets into
wireguard. That's not a nice default to users.  (we've documented
build_dist.sh in our KB)

Restore --extra-small to making a usable build, and add --min for
benchmarking purposes.

Updates #12614

Change-Id: I649e41e324a36a0ca94953229c9914046b5dc497
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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