run: Fix extra LFs and ignored CRs in output

We were previously converting UART output to strings using Python's
universal newline technique, where '\r', '\n' and '\r\n' were all
replaced with '\n'. This caused a few issues; First, telnet would
occasionally output '\r\r\n' which would get converted to 2 newlines,
whereas only one was intended. Secondly, we were swallowing legitimate
'\r's when working at the interactive shell and the cursor would
occasionally get messed up.

Solve this by never converting line endings when converting the binary
data to string data. As a result we must be more carefull with '\r'
since they may now exist in the stream.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>