Add README.md for tools/psa

tools/psa/README.md

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+# PSA Tools
+
+This document will describe the following scripts:
+
+* \_\_init\_\_.py
+* generate_partition_code.py
+* mbed_spm_tfm_common.py
+* generate_mbed_spm_partition_code.py
+* generate_tfm_partition_code.py
+* release.py
+
+## \_\_init\_\_.py
+
+This file holds common function dedicated to help SiP with their post-build logic.
+
+* find_secure_image - Scans a Resource object to find the correct binary of the secure image to merge with the non-secure build.
+
+
+## Code generation scripts
+
+mbed-os holds 2 implementations of PSA:
+* MBED_SPM - Implementation for dual core v7 targets.
+* TF-M - Implementation for v8 targets.
+
+each implementation requires a set of auto generated files describing the secure partitions.
+
+* `generate_partition_code.py` - Generate files for both implementations.
+* `generate_mbed_spm_partition_code.py` - Generate files for BMED_SPM.
+* `generate_tfm_partition_code.py` - Generate files for TF-M.
+*  `mbed_spm_tfm_common.py` - Holds common functions for both.
+
+## Secure image generation
+
+`release.py` is the script assigned with compiling the secure images.
+
+```python
+usage: release.py [-h] [-m MCU]
+
+optional arguments:
+  -h, --help         show this help message and exit
+  -m MCU, --mcu MCU  build for the given MCU
+```
+
+when `MCU ` is not specified the script will compile all the images for all the targets
+
+This script should be ran in following scenarios:
+* Release.
+* Update to files originating in the secure side.
+* Drivers update.
+* PSA updates.