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>

tools/Kconfig

@@ -35,6 +35,11 @@ config TOOLS_FIT_PRINT
 	help
 	  Print the content of the FIT verbosely in the tools builds
 
+config TOOLS_FIT_RSASSA_PSS
+	def_bool y
+	help
+	  Support the rsassa-pss signature scheme in the tools builds
+
 config TOOLS_FIT_SIGNATURE
 	def_bool y
 	help