rsa: rename FIT_RSASSA_PSS to RSASSA_PSS and move symbols under lib/rsa

rsa: rename FIT_RSASSA_PSS to RSASSA_PSS and move symbols under lib/rsa

This renames FIT_RSASSA_PSS symbols to drop the FIT_ prefix to avoid
potential confusion since there's nothing FIT specific to those symbols.

It also isn't really related to booting, so boot/Kconfig is an odd place
for them to live. Since they make sense only in relation with RSA,
simply move them to lib/rsa where it makes more sense for them to
reside.

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