ramips: Make serial console for DIR-300-B1 work again

ramips: Make serial console for DIR-300-B1 work again

Make serial console for DIR-300-B1 work again.

With some recent change, "console=ttyS1,57600" was removed from the
default kernel command line, this adds the necessary mkcmdline
incantation to image/Makefile to add it there for this target.
This is quite likely broken for other targets as well.

Without this the console switches to ttyS0 instead of ttyS1 and
kernel output is missing, and the userspace comes up with 9600 baud
instead of 57600.

Signed-off-by: Tobias Diedrich <ranma+openwrt@tdiedrich.de>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@31556 3c298f89-4303-0410-b956-a3cf2f4a3e73