RFR: 8297348: make CONF=xxx should match if xxx is an exact match

[Magnus Ihse Bursie]

A typical use case for multiple configurations is that you have a release configuration (say `linux-x64`) and a debug configuration (say `linux-x64-debug`). You can now easily select the debug configuration with `CONF=debug`, or both configurations with `CONF=`, but there is no way to select just the release configuration. Instead, workarounds using `SPEC` or `CONF_NAME` (which always does an exact match) is needed.

Instead, we should modify the behavior of `CONF` slightly, so if it gets an exact match, it should behave like `CONF_NAME` and select just that configurations.

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Commit messages:
 - 8297348: make CONF=xxx should match if xxx is an exact match

Changes: https://git.openjdk.org/jdk/pull/11269/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11269&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297348
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11269.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11269/head:pull/11269

PR: https://git.openjdk.org/jdk/pull/11269