RFR: 8325626: Allow selection of non-matching configurations using CONF=!string

[Magnus Ihse Bursie]

A common scenario is for a developer to have two configurations, a product and a debug version, e.g. `linux-x64`and `linux-x64-debug`. To select the latter using `CONF` is just a matter of using `CONF=debug`, but to select the former, the complete name needs to be supplied: `CONF=linux-x64`.

Instead, this patch introduces the use of `!` as a negation character, so `CONF=!debug` would select all configurations that do not match `debug`. 

I also took the time to clarify and correct the documentation on how to use `CONF`.

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Commit messages:
 - 8325626: Allow selection of non-matching configurations using CONF=!string

Changes: https://git.openjdk.org/jdk/pull/17804/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17804&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325626
  Stats: 51 lines in 4 files changed: 30 ins; 1 del; 20 mod
  Patch: https://git.openjdk.org/jdk/pull/17804.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17804/head:pull/17804

PR: https://git.openjdk.org/jdk/pull/17804