auto-mount prevent formating on testboot and testrootfs when image deployment

If master image is Ubuntu LEB, it will mount testboot and testrootfs when bootup. It will prevent format during new image deployment.

Log:
 min/avg/max/mdev = 102.841/102.841/102.841/0.000 ms
root@master:~# mkfs.ext3 -q /dev/disk/by-label/testrootfs -L testrootfs
/dev/disk/by-label/testrootfs is mounted; will not make a filesystem here!
root@master:~# udevadm trigger
root@master:~# mkdir -p /mnt/root
root@master:~# mount /dev/disk/by-label/testrootfs /mnt/root
root@master:~# mxc_ipu mxc_ipu: Channel already disabled 9
mxc_ipu mxc_ipu: Channel already uninitialized 9
 --numeric-owner -C /mnt/root -xzf -s/tmp/tmpx8WE8u/root.tgz |tar
root@master:~# echo linaro > /mnt/root/etc/hostname
root@master:~# umount /mnt/root
root@master:~# mkfs.vfat /dev/disk/by-label/testboot -n testboot
mkfs.vfat 3.0.9 (31 Jan 2010)
mkfs.vfat: unable to open /dev/disk/by-label/testboot
root@master:~# udevadm trigger