syslinux.bbclass: Fix default serial port string

syslinux.bbclass: Fix default serial port string

The default value of SYSLINUX_SERIAL_TTY is not correct.

It should be console=ttyS0,115200 else the boot string generated in
the syslinux menus for the serial choice is not correct.  The kernel
boot parameters will get set to:

/vmlinuz initrd=/initrd LABEL=boot root=/dev/ram0 ttyS0,115200

Note that the above is missing the "console="

The default value will now work the same as the value found in
grub-efi.bbclass.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>