meta: replace deprecated "SERIAL_CONSOLE"

meta: replace deprecated "SERIAL_CONSOLE"

SERIAL_CONSOLE was already deprecated in 2013, yet still some
machine configuration files were using it. This patch replaces
it with SERIAL_CONSOLES, which is the successor.

The default value in systemd-serialgetty.bb can also be safely
transitioned from SERIAL_CONSOLE to SERIAL_CONSOLES, as this
recipe already uses SERIAL_CONSOLES within do_install().

The documentation seems to be already up do date.
beaglebone-yocto.conf in the bsp-guide already uses
SERIAL_CONSOLES. The ref-manual redirects from SERIAL_CONSOLE
to SERIAL_CONSOLES.

[YOCTO #12653]

Signed-off-by: Maciej Pijanowski <maciej.pijanowski@3mdeb.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>