mmc: Enhance get running partition label function

The mount -l command may not show the filesystem label after a code
update since the this filesystem label is part of the image when it
gets built. Therefore determine the running partition via the device
then map it to the partition label (partlabel) which doesn't change.
Also return the label only if found, don't assume that because 'a'
didn't match that the device should be 'b'.

Tested: Verified the right label was found.

Change-Id: Id50223e21e9c6cc280f98f2b928d37a36de48cc9
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/obmc-flash-bmc b/obmc-flash-bmc
index c7f9e14..8e331ca 100644
--- a/obmc-flash-bmc
+++ b/obmc-flash-bmc
@@ -449,24 +449,31 @@
 # The eMMC partition labels for the kernel and rootfs are boot-a/b and rofs-a/b.
 # Return the label (a or b) for the running partition.
 mmc_get_primary_label() {
+  # Get root device /dev/mmcblkpX
   rootmatch=" on / "
-  root="$(mount -l | grep "${rootmatch}")"
-  if [[ "${root}" == *"rofs-a"* ]]; then
+  root="$(mount | grep "${rootmatch}")"
+  root="${root%${rootmatch}*}"
+
+  # Find the device label
+  if [[ $(readlink -f /dev/disk/by-partlabel/rofs-a) == "${root}" ]]; then
     echo "a"
-  else
+  elif [[ $(readlink -f /dev/disk/by-partlabel/rofs-b) == "${root}" ]]; then
     echo "b"
+  else
+    echo ""
   fi
 }
 
 # The eMMC partition labels for the kernel and rootfs are boot-a/b and rofs-a/b.
 # Return the label (a or b) for the non-running partition.
 mmc_get_secondary_label() {
-  rootmatch=" on / "
-  root="$(mount -l | grep "${rootmatch}")"
-  if [[ "${root}" == *"rofs-a"* ]]; then
+  root="$(mmc_get_primary_label)"
+  if [[ "${root}" == "a" ]]; then
     echo "b"
-  else
+  elif [[ "${root}" == "b" ]]; then
     echo "a"
+  else
+    echo ""
   fi
 }