tty: serial: earlycon dependency

parse_options() in drivers/tty/serial/earlycon.c calls uart_parse_earlycon
in drivers/tty/serial/serial_core.c therefore selecting SERIAL_EARLYCON
should automatically select SERIAL_CORE, otherwise will result in symbol
not found error during linking if SERIAL_CORE is not configured as builtin

Fixes: 9aac5887 ("tty/serial: add generic serial earlycon")
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Link: https://lore.kernel.org/r/20200828123949.2642-1-ztong0001@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/Kconfig

@@ -8,6 +8,7 @@ menu "Serial drivers"
 
 config SERIAL_EARLYCON
 	bool
+	depends on SERIAL_CORE
 	help
 	  Support for early consoles with the earlycon parameter. This enables
 	  the console before standard serial driver is probed. The console is