utils: add umount_all function

Add utility function to unmount any mountpoint present in the /mnt
folder.

This function is a simplified version of the legacy bash function
ogUnmountAll used in several operations.

src/utils/fs.py

@@ -69,6 +69,15 @@ def umount(target):
     return not proc.returncode
 
 
+def umount_all():
+    """
+    Umounts all mountpoints present in the /mnt folder.
+    """
+    for path in ['/mnt/'+child for child in os.listdir('/mnt/')]:
+        if os.path.ismount(path):
+            umount(path)
+
+
 def get_usedperc(mountpoint):
     """
     Returns percetage of used filesystem as decimal number.