tools: scripts: mbed: update build path

[amiclaus]

Pull Request Description

The MBED_OS_BUILD_DIRECTORY holds the build files that the mbed platform generates. These files should be placed in the same manner as all the other platform, respectively under the BUILD_DIR environment path.

This patch fixes the CI builds for the mbed platform and also makes sure that all the build generated files are kept under the same path when building manually a project under the projects folder.

PR Type

• [x] Bug fix (change that fixes an issue)
• [ ] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• [x] I have followed the Coding style guidelines
• [x] I have performed a self-review of the changes
• [ ] I have commented my code, at least hard-to-understand parts
• [ ] I have build all projects affected by the changes in this PR
• [ ] I have tested in hardware affected projects, at the relevant boards
• [x] I have signed off all commits from this PR
• [ ] I have updated the documentation (wiki pages, ReadMe etc), if applies