As the initial output of picocom is hardly ever needed, disable it by
default to reduce the log noise. Whenever needed, the output can still
be enabled by specifying the run tool's --log-serial-cmd.
While booting up, some devices send garbage over the serial connection.
This can result in an unexpected EOF event within expect(1). A filter
program may be specified with '--log-serial-filter' to circumvent this
problem. Then, the output of the serial program is piped through
the filter, which may sanitize the character stream.
Fixes#1395.
