scons: show sources and targets when building, and colorize output.

I like the brevity of Ali's recent change, but the ambiguity of
sometimes showing the source and sometimes the target is a little
confusing.  This patch makes scons typically list all sources and
all targets for each action, with the common path prefix factored
out for brevity.  It's a little more verbose now but also more
informative.

Somehow Ali talked me into adding colors too, which is a whole
'nother story.

src/arch/SConscript

@@ -118,7 +118,7 @@ def isa_desc_action_func(target, source, env):
     cpu_models = [CpuModel.dict[cpu] for cpu in models]
     parser = isa_parser.ISAParser(target[0].dir.abspath, cpu_models)
     parser.parse_isa_desc(source[0].abspath)
-isa_desc_action = MakeAction(isa_desc_action_func, " [ISA DESC] $STRIP_SOURCE")
+isa_desc_action = MakeAction(isa_desc_action_func, Transform("ISA DESC", 1))
 
 # Also include the CheckerCPU as one of the models if it is being
 # enabled via command line.