Blackfin: jtag-console: handle newline stuffing

Serial devices currently have to manually stuff \r after every \n found,
but this is a bit more difficult with the jtag console since we process
everything in chunks of 4 bit.  So we have to scan & stuff the whole
string rather than what most serial drivers do which is output on a byte
per byte basis.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>