feat: add optional BUILD_DIR Makefile variable to place build outputs in a separate directory

[DanielSRS]

Introduce an optional variable so build artifacts can be placed in a dedicated directory

This pull request updates the Makefile to add support for specifying an output build directory via the new BUILD_DIR variable. All build artifacts, including binaries, APKs, and intermediate files, are now placed in the specified BUILD_DIR. This improves build organization and makes it easier to manage output files.

What this PR does

• Adds an optional Makefile variable (BUILD_DIR) that, when set, causes all output files to be produced under the given directory.
• Defaults BUILD_DIR to the root directory so that if it is not set the Makefile behaves exactly as before (artifacts in repo root).
• The only file that is still generated at the root is the .keystore file
• The behavior change is opt-in

[cnlohr]

Looks good.