run: avoid buffering of output for expect

Due to commit "run: relax IP power plug recognition + serial EOF",
when piping the serial command through 'tr', some characters might
get buffered, thereby preventing some run scripts to finish correctly.
This commit removes the 'tr' hack. Instead, to circumvent the 'expect'
problem, which kills under special conditions spawned childs used to
obtain serial line content, whenever EOF of the serial command is
recognized during the boot phase, the child process gets re-spawned.