Replace references to the deprecated functions `psa_generate_key_ext()` and
`psa_key_derivation_output_key_ext()` by their replacements
Implement `psa_generate_key_custom()` and
`psa_key_derivation_output_key_custom()`.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>