samples: psa_crypto: fix setting of TF-M's CRYPTO_ENGINE_BUF_SIZE

It used to be set as a CMake variable, but TF-M's build system does
not look at such a variable.
Instead, define an additional configuration header file that defines
CRYPTO_ENGINE_BUF_SIZE, and pass it to TF-M's build system.

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