创建，添加sdram

rt-thread/libcpu/risc-v/SConscript

@@ -0,0 +1,26 @@
+# RT-Thread building script for bridge
+
+import os
+from building import *
+
+Import('rtconfig')
+
+cwd   = GetCurrentDir()
+group = []
+list  = os.listdir(cwd)
+
+# add common code files
+if   rtconfig.CPU == "virt64" :
+    group = group
+elif rtconfig.CPU == "c906" :
+    group = group
+else :
+    group = group + SConscript(os.path.join('common', 'SConscript'))
+
+# cpu porting code files
+if  rtconfig.CPU == "c906":
+    group = group + SConscript(os.path.join(rtconfig.VENDOR, rtconfig.CPU, 'SConscript'))
+elif rtconfig.CPU in list:
+    group = group + SConscript(os.path.join(rtconfig.CPU, 'SConscript'))
+
+Return('group')