add logtrace component

Logtrace is a component that could output the log into either a device
or a file. It has the ability to filter the log messages according to a
pre-module level. Define RT_USING_LOGTRACE in rtconfig.h if you want to
have a try.

components/logtrace/SConscript

@@ -0,0 +1,9 @@
+from building import *
+
+cwd     = GetCurrentDir()
+src     = Glob('*.c')
+CPPPATH = [cwd]
+
+group = DefineGroup('LogTrace', src, depend = ['RT_USING_LOGTRACE'], CPPPATH = CPPPATH)
+
+Return('group')