Blackfin: bump up default JTAG console timeout

The debug tools that interface with the other side of the JTAG console
got much slower when generalizing things, so bump up the default timeout
value on the U-Boot side to cope.  Hopefully at some point we can improve
the debug tools to speed things back up.

Signed-off-by: Vivi Li <vivi.li@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>