Move MBEDTLS_CIPHER_NULL_CIPHER to TLS and maybe rename

[gilles-peskine-arm]

MBEDTLS_CIPHER_NULL_CIPHER controls whether null cipher suites are enabled in TLS. For historical reasons due to how null cipher suites were implemented (using functionality from cipher.h), it was a crypto option, and at the time of the repo split, it went into TF-PSA-Crypto. However, it has no useful effect on crypto (it just enables some code in cipher.c which is not reachable from any public API of TF-PSA-Crypto 1.0). But it is still needed in TLS, to control whether null cipher suites are enabled.

The goal of this issue is:

• Remove MBEDTLS_CIPHER_NULL_CIPHER from the list of selectable options in the TF-PSA-Crypto configuration file.
• Optionally remove all code and tests guarded by MBEDTLS_CIPHER_NULL_CIPHER. This can wait until after the 1.0 release.
• Add MBEDTLS_CIPHER_NULL_CIPHER to mbedts_config.h in Mbed TLS.
• Optionally, rename MBEDTLS_CIPHER_NULL_CIPHER to MBEDTLS_SSL_NULL_CIPHER (name TBD).
• Make sure that null cipher suites are still tested (if they aren't, outcome analysis should complain).