RFR: build: add convenience target for bots

[Erik Helin]

Hi all,

please review this patch that adds a convenience target for building and extracting a image for the bots. A developer can now just run `sh gradlew bots` (or `make bots`) and find an extracted image under `bots/bin`.

Testing:
- [x] Tested locally on Linux x64

Thanks,
Erik

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Commit messages:
 - build: add convenience target for bots

Changes: https://git.openjdk.java.net/skara/pull/1000/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1000&range=00
  Stats: 18 lines in 2 files changed: 17 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/1000.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1000/head:pull/1000

PR: https://git.openjdk.java.net/skara/pull/1000