SCons: Reduce and cleanup verbose output for SCU builds

Verbose output is meant for debugging the SCU mode itself and can be
triggered by changing the `_verbose` bool manually.

Prefix all prints with "SCU:" for context, and print the processed
folders all at once instead of when adding the sources.

SConstruct

@@ -565,7 +565,7 @@ if selected_platform in platform_list:
         if read_scu_limit != 0:
             max_includes_per_scu = read_scu_limit
 
-        methods.set_scu_folders(scu_builders.generate_scu_files(env["verbose"], max_includes_per_scu))
+        methods.set_scu_folders(scu_builders.generate_scu_files(max_includes_per_scu))
 
     # Must happen after the flags' definition, as configure is when most flags
     # are actually handled to change compile options, etc.