syslinux.bbclass: Add a default serial console option and real boot menu support

syslinux.bbclass: Add a default serial console option and real boot menu support

The previous syslinux menu code did not support using both a serial
and vga console, but this has worked for years in syslinux so there is
no reason not to take advantage of it.  The previous menu looked like:

-------------------------------------------------------
Linux Boot Menu
The following targets are available on this image:

             boot: None
             install: None
-------------------------------------------------------

This commit makes it look something more like a traditional grub menu
on both the serial console and vga console as well as providing the
option to continue on using either the serial or vga console with the
correct kernel arguments.

You can see the screen shots attached to the bugzilla.

https://bugzilla.yoctoproject.org/show_bug.cgi?id=3944

[ YOCTO #3944 ]

(From OE-Core rev: 7032d559c04bae09e6b7c39ddeb1bf35acc0584c)

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>