storage: flash_map: replace TinyCrypt by PSA

As part of ongoing work to move away from TinyCrypt and towards PSA
(#43712), introduce a PSA option and remove the TinyCrypt one for the
SHA-256 implementation.

The Mbed TLS implementation is modified to use `mbedtls_sha256`
directly for smaller code size.

As of now the implementation defaults to PSA only if TF-M is enabled
because a dependency loop happens if using `PSA_CRYPTO_CLIENT` as a
condition in `FLASH_AREA_CHECK_INTEGRITY_BACKEND`.

A test case is added for the PSA implementation, and an NS platform is
added to the base test case to verify the compilation on a TF-M-enabled
platform.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>