systemd-serialgetty: allow baud rate overriding

systemd-serialgetty: allow baud rate overriding

In case a getty is required on a UART which is not being
used as the kernel console, the current agetty invocation
fails to obey the baud rate configured through the
SERIAL_CONSOLES variable because it uses --keep-baud.

(From OE-Core master rev: b54b73834e73d55de1038b55d0a4d7f49cda52d0)

Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>