mgmt: updatehub: 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.

The reliance of mgmt/updatehub on storage/flash_map's configuration
(`FLASH_AREA_CHECK_INTEGRITY_BACKEND`) is removed.
The choice of which implementation to use is made automatically,
based on whether a PSA API provider is present (`PSA_CRYPTO_CLIENT`).

This commit also add a test case with PSA (based on Mbed TLS)
in samples/subsys/mgmt/updatehub/sample.yaml.

Signed-off-by: Tomi Fontanilles <tomi.fontanilles@nordicsemi.no>
Signed-off-by: Valerio Setti <vsetti@baylibre.com>