Clarify documentation
Comment on target connection
When printing an executed command, don't print quote every parameter

Only parameters containing spaces semicolons etc. are quoted.
Use Expect package to communicate with the target over serial line

This simplifies the code and unifies the target communication. Now, this
is used for communication with the IP relay, host serial line and qemu
subprocess. More is expected to come.
Initialize all possible target connections at one place

i.e. one large if/elif/else block.

Also modularize target reset/poweron/off.
Fix warnings when invoked with no script
Add --stty option to specify serial line configuration