syslinux.bbclass: Fix hard coding of console=tty*

syslinux.bbclass: Fix hard coding of console=tty*

The SYSLINUX_SERIAL variable was hard coded and occasionally needs to
be different for the kernel argument vs the syslinux argument.

In the auto-generated boot mode console=tty0 was hard coded, and this
is not needed at all, and causes problems in some cases if a end user
wanted to change the console=... via the kernel boot argument APPEND
mechanism.  The default can be forced with SYSLINUX_DEFAULT_CONSOLE
for systems that need a special specification to enable the frame
buffer instead of a serial port.

[YOCTO #3944]

(From OE-Core rev: cf2fba810a8a59cff71bf2c12e516e9080146604)

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>