gen_initramfs: use systemd-udevd if udevd is not available

This fixes systemd-207 support and bug #485094

defaults/initrd.defaults

@@ -55,7 +55,10 @@ ROOT_LINKS='bin sbin lib lib32 lib64 boot usr opt emul'
 ROOT_TREES='etc root home var'
 KSUFF='.ko'
 
+# systemd-207 dropped /sbin/udevd
 UDEVD='/sbin/udevd'
+[ ! -e "${UDEVD}" ] && UDEVD='/usr/lib/systemd/systemd-udevd'
+
 MDEVD='/sbin/mdev'
 INITRD_SPLASH='/etc/initrd.splash'
 PLYMOUTHD_BIN='/sbin/plymouthd'

gen_initramfs.sh

@@ -612,9 +612,13 @@ append_udev() {
         fi
     done
 
+    # systemd-207 dropped /sbin/udevd
+    local udevd_bin=/sbin/udevd
+    [ ! -e "${udevd_bin}" ] && udevd_bin=/usr/lib/systemd/systemd-udevd
+
     # Copy binaries
     copy_binaries "${TEMP}/initramfs-udev-temp" \
-        /sbin/udevd /bin/udevadm "${udev_dir}/scsi_id" \
+        "${udevd_bin}" /bin/udevadm "${udev_dir}/scsi_id" \
         "${udev_dir}/ata_id" "${udev_dir}/mtd_probe"
 
     cd "${TEMP}/initramfs-udev-temp/"