For the MD_ROOT option don't inject /dev/md0 as root dev when ROOTDEVNAME

is defined explicitly. It's kinda pointless and results in extra step in
boot sequence which is not really needed, i.e.:

md0: Embedded image 1331200 bytes at 0x8038b7b4
Trying to mount root from ufs:/dev/md0 []...
Mounting from ufs:/dev/md0 failed with error 22.
Trying to mount root from ufs:md0.uzip []...
warning: no time-of-day clock registered, system time will not be set accurately
start_init: trying /sbin/init

sys/dev/md/md.c

@@ -58,6 +58,7 @@
  * From: src/sys/dev/vn/vn.c,v 1.122 2000/12/16 16:06:03
  */
 
+#include "opt_rootdevname.h"
 #include "opt_geom.h"
 #include "opt_md.h"
 
@@ -1732,7 +1733,7 @@ md_preloaded(u_char *image, size_t length, const char *name)
 	sc->pl_ptr = image;
 	sc->pl_len = length;
 	sc->start = mdstart_preload;
-#ifdef MD_ROOT
+#if defined(MD_ROOT) && !defined(ROOTDEVNAME)
 	if (sc->unit == 0)
 		rootdevnames[0] = MD_ROOT_FSTYPE ":/dev/md0";
 #endif

sys/modules/md/Makefile

@@ -3,6 +3,6 @@
 .PATH: ${.CURDIR}/../../dev/md
 
 KMOD=	geom_md
-SRCS=	md.c opt_md.h opt_geom.h vnode_if.h
+SRCS=	md.c opt_md.h opt_geom.h opt_rootdevname.h vnode_if.h
 
 .include <bsd.kmod.mk>