Based on:

> Forward-port the old mdstart tool from the Gentoo Busybox-1.1.3.
> Only fires the RAID_AUTORUN ioctl on existing /dev/md nodes.

diff -pruN a/include/applets.src.h mdstart/include/applets.src.h
--- a/include/applets.src.h	2011-01-20 01:08:05.470632138 +0100
+++ mdstart/include/applets.src.h	2011-01-20 01:09:13.198003320 +0100
@@ -235,6 +235,7 @@ IF_MAN(APPLET(man, _BB_DIR_SBIN, _BB_SUI
 IF_MATCHPATHCON(APPLET(matchpathcon, _BB_DIR_USR_SBIN, _BB_SUID_DROP))
 IF_MD5SUM(APPLET_NOEXEC(md5sum, md5_sha1_sum, _BB_DIR_USR_BIN, _BB_SUID_DROP, md5sum))
 IF_MDEV(APPLET(mdev, _BB_DIR_SBIN, _BB_SUID_DROP))
+IF_MDSTART(APPLET(mdstart, _BB_DIR_SBIN, _BB_SUID_DROP))
 IF_MICROCOM(APPLET(microcom, _BB_DIR_USR_BIN, _BB_SUID_DROP))
 IF_MKDIR(APPLET_NOFORK(mkdir, mkdir, _BB_DIR_BIN, _BB_SUID_DROP, mkdir))
 IF_MKFS_VFAT(APPLET_ODDNAME(mkdosfs, mkfs_vfat, _BB_DIR_SBIN, _BB_SUID_DROP, mkfs_vfat))
diff -pruN a/include/usage.src.h mdstart/include/usage.src.h
--- a/include/usage.src.h	2010-12-21 06:29:45.000000000 +0200
+++ mdstart/include/usage.src.h	2011-01-12 21:29:47.000000000 +0200
@@ -847,6 +847,11 @@ INSERT
        "$ dirname /tmp/foo/\n" \
        "/tmp\n"
 
+#define mdstart_trivial_usage \
+	"{[PARTITION] MD-NODE}..."
+#define mdstart_full_usage \
+	"Run the RAID_AUTORUN ioctl on the given MD number"
+
 #define dmesg_trivial_usage \
        "[-c] [-n LEVEL] [-s SIZE]"
 #define dmesg_full_usage "\n\n" \
diff -pruN a/util-linux/Config.src mdstart/util-linux/Config.src
--- a/util-linux/Config.src	2010-12-20 02:41:27.000000000 +0200
+++ mdstart/util-linux/Config.src	2011-01-12 21:30:09.000000000 +0200
@@ -456,6 +456,13 @@ config FEATURE_MDEV_LOAD_FIRMWARE
 	  /lib/firmware/ and if it exists, send it to the kernel for
 	  loading into the hardware.
 
+config MDSTART
+	bool "mdstart"
+	default n
+	help
+	  Allows you to autostart /dev/md devices if using an initramfs to
+	  boot.
+
 config MKSWAP
 	bool "mkswap"
 	default y
diff -pruN a/util-linux/Kbuild.src mdstart/util-linux/Kbuild.src
--- a/util-linux/Kbuild.src	2010-12-20 02:41:27.000000000 +0200
+++ mdstart/util-linux/Kbuild.src	2011-01-12 21:30:09.000000000 +0200
@@ -24,6 +24,7 @@ lib-$(CONFIG_HWCLOCK)           += hwclo
 lib-$(CONFIG_IPCRM)             += ipcrm.o
 lib-$(CONFIG_IPCS)              += ipcs.o
 lib-$(CONFIG_LOSETUP)           += losetup.o
+lib-$(CONFIG_MDSTART)          += mdStart.o
 lib-$(CONFIG_LSPCI)             += lspci.o
 lib-$(CONFIG_LSUSB)             += lsusb.o
 lib-$(CONFIG_MDEV)              += mdev.o
diff -pruN a/util-linux/mdStart.c mdstart/util-linux/mdStart.c
--- a/util-linux/mdStart.c	1970-01-01 03:00:00.000000000 +0300
+++ mdstart/util-linux/mdStart.c	2011-01-12 21:30:09.000000000 +0200
@@ -0,0 +1,59 @@
+/*
+ * Linux 2.6(+) RAID Autostarter
+ *
+ * Copyright (C) 2005 by Tim Yamin <plasmaroo@gentoo.org> <plasm@roo.me.uk>
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
+ *
+ */
+
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <sys/ioctl.h>
+#include <linux/major.h>
+#include <linux/raid/md_u.h>
+
+extern int
+mdstart_main(int argc, char *argv[])
+{
+	int i, fd, part = 0, retval = 0;
+
+	if(argc < 2)
+	{
+		bb_show_usage();
+	}
+
+	for(i = 1; i < argc; i++)
+	{
+		if(sscanf(argv[i], "%d", &part) == 1)
+			continue;
+
+		fd = open(argv[i], 0, 0);
+		if (fd >= 0)
+		{
+			ioctl(fd, RAID_AUTORUN, part);
+			close(fd);
+		} else
+		{
+			printf("Error: Failed to open %s!\n", argv[i]);
+			retval=1;
+		}
+
+		part = 0;
+	}
+
+	return retval;
+}