psa: Refactor psa_internal_export_key to use slot, rather than key
psa: Refactor psa_internal_export_key to use slot, rather than key

When generating keys that have persistent lifetime, we will need
the keys to be in the exported format to save to persistent storage.
This refactoring to separate checking the slots usage from the
exporting of the key data will be necessary for using
psa_internal_export_key in psa_generate_key.

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