创建，添加sdram

rt-thread/components/libc/posix/signal/SConscript

@@ -0,0 +1,13 @@
+# RT-Thread building script for component
+
+from building import *
+
+cwd = GetCurrentDir()
+src = Glob('*.c') + Glob('*.cpp')
+CPPPATH = [cwd]
+
+group = DefineGroup('POSIX', src,
+    depend = ['RT_USING_SIGNALS', 'RT_USING_PTHREADS'],
+    CPPPATH = CPPPATH)
+
+Return('group')