amd64 loader: plug hard hang with serial console enabled

The hang basically bricks a physical box and it can be recovered
only if you are able to boot from alternate media.  This isn't a
perfect fix, but throw it in before loader experts decide on
proper one.

Submitted by:	whu
Fixes:		927358d

(cherry picked from commit 4358928)

stand/efi/loader/efiserialio.c

@@ -265,6 +265,8 @@ comc_probe(struct console *sc)
 	 */
 	env = getenv("smbios.bios.version");
 	if (env == NULL || strncmp(env, "Hyper-V", 7) != 0) {
+		/* Disable being seen as "comconsole". */
+		comconsole.c_name = "efiserialio";
 		return;
 	}
 #endif