disk: ability to grow existing partitions

Apply the minimum size specified in the file system customizations 
also to existing partitions. Specifically, this now allows to set
a minimum size also for the root partition.

internal/disk/customizations.go

@@ -34,7 +34,14 @@ func CreatePartitionTable(
 
 	for _, m := range mountpoints {
 		sectors := table.BytesToSectors(m.MinSize)
-		if m.Mountpoint != "/" {
+
+		// if we already have a partition ensure that the
+		// size is at least the requested size, otherwise
+		// create a new filesystem with that size
+		part := table.FindPartitionForMountpoint(m.Mountpoint)
+		if part != nil {
+			part.EnsureSize(sectors)
+		} else {
 			table.CreateFilesystem(m.Mountpoint, sectors)
 		}
 	}