arch/x86: Enable postcar console

Add a Kconfig value to enable the console during postcar.  Add a call
to console_init at the beginning of the postcar stage in exit_car.S.

TEST=Build and run on Galileo Gen2

Change-Id: I66e2ec83344129ede2c7d6e5627c8062e28f50ad
Signed-off-by: Lee Leahy <leroy.p.leahy@intel.com>
Reviewed-on: https://review.coreboot.org/16001
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/console/init.c

@@ -30,7 +30,7 @@ int console_log_level(int msg_level)
 	return (console_loglevel >= msg_level);
 }
 
-void console_init(void)
+asmlinkage void console_init(void)
 {
 #if !defined(__PRE_RAM__)
 	console_loglevel = CONFIG_DEFAULT_CONSOLE_LOGLEVEL;