Fix race condition when running generate_psa_tests.py
Fix race condition when running generate_psa_tests.py

Fix a race condition in parallel builds: when generating *.data files with
generate_psa_tests.py, make instantiated the recipe once per output file,
potentially resulting in multiple instances of generate_psa_tests.py running
in parallel. This not only was inefficient, but occasionally caused the
output to be corrupted (https://github.com/ARMmbed/mbedtls/issues/4773). Fix
this by ensuring the recipe only runs once.

Signed-off-by: Gilles Peskine <f805f64266d288fc5467baa7be6cd0ff366f477b@arm.com>

Mbed-TLS/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls