tty: goldfish: Enable 'earlycon' only if built-in

Commit 3840ed95 ("tty: goldfish: Implement support for kernel
'earlycon' parameter") breaks an allmodconfig config on x86:
|  LD      vmlinux.o
|  MODPOST vmlinux.o
|drivers/tty/serial/earlycon.o: In function `parse_options':
|drivers/tty/serial/earlycon.c:97: undefined reference to `uart_parse_earlycon'
|Makefile:1005: recipe for target 'vmlinux' failed

earlycon.c::parse_options() invokes uart_parse_earlycon() from serial_core.c
which is compiled=m because GOLDFISH_TTY itself (and most others) are =m.
To avoid that, I'm adding the _CONSOLE config option which is selected if the
GOLDFISH module itself is =y since it doesn't need the early bits for the =m
case (other drivers do the same dance).
The alternative would be to move uart_parse_earlycon() from
serial_core.c to earlycon.c (we don't have that many users of that
function).

Fixes: 3840ed95 ("tty: goldfish: Implement support for kernel
       'earlycon' parameter")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Miodrag Dinic <miodrag.dinic@mips.com>
Acked-by: Miodrag Dinic <miodrag.dinic@mips.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>