BuildEnv: Eliminate RUBY as build environment variable

This patch replaces RUBY with PROTOCOL in all the SConscript files as
the environment variable that decides whether or not certain components
of the simulator are compiled.

src/mem/protocol/SConscript

@@ -38,7 +38,7 @@ from SCons.Scanner import Classic
 
 Import('*')
 
-if not env['RUBY']:
+if env['PROTOCOL'] == 'None':
     Return()
 
 protocol_dir = Dir('.')