boot: fix incorrect dependency of FIT_RSASSA_PSS

boot: fix incorrect dependency of FIT_RSASSA_PSS

This padding has nothing to do with FIT except that we can make use of
it when verifying the FIT signatures.

This padding can also be used to verify the signature "manually" e.g. by
calling rsa_verify_hash() directly with an embedded public key.

Additionally, this padding is only useful if RSA (and specifically
RSA_VERIFY) is enabled otherwise it's not used.
The only other place it's used is in rsa-sign.c which is only built for
the host tools and handled by TOOLS_FIT_RSASSA_PSS symbol instead, so no
need to care for that one.

Finally, the FIT_SIGNATURE dependency also wasn't enough because it only
implies RSA_VERIFY, meaning it can be disabled and still have
FIT_RSASSA_PSS enabled.

So add a dependency on RSA_VERIFY and reword the input prompt.

Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>