As part of ongoing work to move away from TinyCrypt and towards PSA
(#43712), make fs_mgmt use either PSA (when available) or MbedTLS
(as a fallback) for SHA-256.
The use of PSA is guarded by CONFIG_MBEDTLS_PSA_CRYPTO_CLIENT
which requires a locally-built PSA core for devices without TF-M.
Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>
6002abe519