image: Rename CONFIG_FIT_ENABLE_RSASSA_PSS_SUPPORT

Drop the ENABLE and SUPPORT parts of this, which are redundant.

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