add SQLite 3.8.1 amalgamation source code

components/external/sqlite/README

+# SQLite on RT-Thread
+
+## 简介
+初始版本基于SQLite 3.8.1版本，使用混合单文件结构源代码

components/external/sqlite/SConscript

+Import('rtconfig')
+import os
+from building import *
+
+cwd = GetCurrentDir()
+src	= Glob('*.c')
+CPPPATH = [cwd, str(Dir('#'))]
+
+group = DefineGroup('sqlite', src, depend = ['RT_USING_SQLITE'], CPPPATH = CPPPATH)
+
+Return('group')