petitboot: Update mdadm path

Commit 0500d1a "mdadm: move from /usr/sbin to /sbin" in Buildroot moved
the location of mdadm; update our udev scripts to reflect the change.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
diff --git a/openpower/package/petitboot/63-md-raid-arrays.rules b/openpower/package/petitboot/63-md-raid-arrays.rules
index 385a8da..92aec36 100644
--- a/openpower/package/petitboot/63-md-raid-arrays.rules
+++ b/openpower/package/petitboot/63-md-raid-arrays.rules
@@ -17,7 +17,7 @@
 ATTR{md/array_state}=="|clear|inactive", ENV{SYSTEMD_READY}="0", GOTO="md_end"
 LABEL="md_ignore_state"
 
-IMPORT{program}="/usr/sbin/mdadm --detail --export $devnode"
+IMPORT{program}="/sbin/mdadm --detail --export $devnode"
 ENV{DEVTYPE}=="disk", ENV{MD_NAME}=="?*", SYMLINK+="disk/by-id/md-name-$env{MD_NAME}", OPTIONS+="string_escape=replace"
 ENV{DEVTYPE}=="disk", ENV{MD_UUID}=="?*", SYMLINK+="disk/by-id/md-uuid-$env{MD_UUID}"
 ENV{DEVTYPE}=="disk", ENV{MD_DEVNAME}=="?*", SYMLINK+="md/$env{MD_DEVNAME}"