lib: rsa: Update function padding_pss_verify (any-salt)

Modify function to support any salt length instead of max
length only. Function now detects salt length by parsing
the content of db buffer. Note that it works with (but is
not limited to) zero-length, digest-length and max-length

Signed-off-by: SESA644425 <gioja.hermann@non.se.com>
Reviewed-by: Simon Glass <sjg@chromium.org>