This commit adds support joiner discerner in CLI. The format of joiner
discerner is `number/length`. For example `0xabc/12` means the discerner
value is `0xabc` and the length is `12`.
This makes expect run all cli-*.exp and posix-*.exp tests in RCP mode.
New scripts should use the function spawn_node in _common.exp to spawn
new processes. This is compatible with both simulation mode and RCP
mode.
This puts the scripts setting up Thread networks into separate files
so that they can be reused by multiple test scripts. After this we can
split the tests in cli-2-nodes.exp into multiple files without having
to repeat codes in each file.
Yakun Xu