Add a capability for mainboard-specific posting.

Some mainboards have really nice capabilities for posting, beyond
simple POST cards. Further, some can not use a POST card. This
change defines a weak symbol (mainboard_post) that can be overridden
by a real mainboard_post function.

If, for example, you'd like to do something fancy before the payload starts,
you can add this to mainboard.c:

void mainboard_post(u8 value)
{
	switch(value){
		case POST_TIME_TO_PARTY: some_fancy_lights();
		break;
	}
}

Maybe the post function should be an entry in the device. We're beginning to over-use
weak symbols.

BUG=None

TEST=Build and boot a google chromebook. Observe that it still works. Use it to drive
some pretty lights.

Change-Id: I3512d2ec34a66c747287191851c3f68b6a7cc1b2
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-on: http://review.coreboot.org/1397
Tested-by: build bot (Jenkins)

src/include/console/console.h

@@ -69,6 +69,8 @@ extern int console_loglevel;
 #ifndef __ROMCC__
 void console_init(void);
 void post_code(u8 value);
+/* this function is weak and can be overridden by a mainboard function. */
+void mainboard_post(u8 value);
 void __attribute__ ((noreturn)) die(const char *msg);
 int do_printk(int msg_level, const char *fmt, ...) __attribute__((format(printf, 2, 3)));