Move pkgconfig.sh to the framework

[mpg]

Move tests/scripts/pkgconfig.sh to the framework.

This script has diverged between development and 3.6. This task is to:

• reconcile the two branches;
• move the script to the framework;
• adapt the path everywhere the script is invoked.

Note: this task is only about mbedtls and the framework. This script is supposed to be already adapted to tf-psa-crypto; we just need to take care not to undo that while reconciling the two mbedtls branches - this will likely mean aligning on the development version in most cases, but please consider for each case if that is indeed the right thing to do.