update: Save files using the same mount path as init

Use the full /run/initramfs/rw path for the mount point of the
read-write filesystem.

This prevents creating and directories in / when it is invoked
before shutdown.

Signed-off-by: Milton Miller <miltonm@us.ibm.com>
diff --git a/common/recipes-phosphor/obmc-phosphor-initfs/files/obmc-update.sh b/common/recipes-phosphor/obmc-phosphor-initfs/files/obmc-update.sh
index 1a11953..e580561 100755
--- a/common/recipes-phosphor/obmc-phosphor-initfs/files/obmc-update.sh
+++ b/common/recipes-phosphor/obmc-phosphor-initfs/files/obmc-update.sh
@@ -55,7 +55,7 @@
 rwopts=rw
 rorwopts=ro${rwopts#rw}
 
-rwdir=rw
+rwdir=/run/initramfs/rw
 upper=$rwdir/cow
 save=/run/save/${upper##*/}