[NET] Add telnet app to netutils.

components/net/netutils/README.md

+Some net utils which using standard interface such as BSD Sockets.
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components/net/netutils/SConscript

+from building import *
+
+cwd = GetCurrentDir()
+src = Glob('*.c')
+
+CPPPATH = [cwd]
+
+group = DefineGroup('LwIP', src, depend = ['RT_USING_NETUTILS'], CPPPATH = CPPPATH)
+
+Return('group')
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