Relocate TF-M repositories to a shorter path

[LDong-Arm]

Fixes: #25

The previous path

mbed-os/platform/FEATURE_EXPERIMENTAL_API/FEATURE_PSA/TARGET_TFM/TARGET_TFM_LATEST/TARGET_IGNORE/

where we clone trusted-firmware-m, tf-m-tests and psa-arch-tests into is way too long:

• This often exceeds the default maximum path length allowed by Windows, resulting in fetch and build failures.
• The path is hard for developers to locate, if they want to make code changes.
• The cloned repositories are not directly used by Mbed OS - it's mbed-os-tf-m-regression-tests which builds those repositories.

[LDong-Arm]

I'm letting Circle CI and Travis test the build, so I don't need to do it locally.