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

[Erik Joelsson]

On Tue, 7 Oct 2025 22:04:56 GMT, Mikael Vidstedt <mikael at openjdk.org> wrote:

> 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

Marked as reviewed by erikj (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27679#pullrequestreview-3312155332