mb/up/squared: Add kernel cmdline parameters

Since ttyS0 isn't used for UART0, configure ttyS4 as default

Change-Id: Ia0469226253b08328807d5401c05633296e43d22
Signed-off-by: Felix Singer <felix.singer@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/33785
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/up/squared/Kconfig

@@ -65,6 +65,10 @@ config MAX_CPUS
 	int
 	default 2
 
+config LINUX_COMMAND_LINE
+	string
+	default "console=ttyS4,115200 earlyprintk=ttyS4,115200,keep" if PAYLOAD_LINUXBOOT && UART_FOR_CONSOLE=0
+
 config UART_FOR_CONSOLE
 	int
 	default 0