image: Use Kconfig to enable FIT_RSASSA_PSS on host

Add a host Kconfig for FIT_RSASSA_PSS. With this we can use
CONFIG_IS_ENABLED(FIT_RSASSA_PSS) directly in the host build, so drop the
forcing of this in the image.h header.

Drop the #ifdef around padding_pss_verify() too since it is not needed.
Use the compiler to check the config where possible, instead of the
preprocessor.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>