build scripts: Made minor modifications to reduce build overhead time.

1. --implicit-cache behavior is default.
2. makeEnv in src/SConscript is conditionally called.
3. decider set to MD5-timestamp
4. NO_HTML build option changed to SLICC_HTML (defaults to False)

src/mem/protocol/SConscript

@@ -69,7 +69,7 @@ def slicc_emitter(target, source, env):
     slicc = SLICC(filepath, verbose=False)
     slicc.process()
     slicc.writeCodeFiles(protocol_dir.abspath)
-    if not env['NO_HTML']:
+    if env['SLICC_HTML']:
         slicc.writeHTMLFiles(html_dir.abspath)
 
     target.extend([protocol_dir.File(f) for f in sorted(slicc.files())])
@@ -82,7 +82,7 @@ def slicc_action(target, source, env):
     slicc = SLICC(filepath, verbose=True)
     slicc.process()
     slicc.writeCodeFiles(protocol_dir.abspath)
-    if not env['NO_HTML']:
+    if env['SLICC_HTML']:
         slicc.writeHTMLFiles(html_dir.abspath)
 
 slicc_builder = Builder(action=MakeAction(slicc_action, Transform("SLICC")),