RFR: 8369328: Use uppercase variable names in the devkit makefiles

[Mikael Vidstedt]

Bring the devkit makefiles closer to the naming convention used in the JDK build system by using uppercase variable names.

Testing:

* Built devkit on linux-x64
* Verified that the JDK builds with the new devkit

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Commit messages:
 - 8369328: Use uppercase variable names in the devkit makefiles

Changes: https://git.openjdk.org/jdk/pull/27679/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27679&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8369328
  Stats: 98 lines in 2 files changed: 0 ins; 0 del; 98 mod
  Patch: https://git.openjdk.org/jdk/pull/27679.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27679/head:pull/27679

PR: https://git.openjdk.org/jdk/pull/27679