Add support for custom persistence config files

[okmans]

Added the ability to specify a persistence config file with custom mount points.

Tests

1. Test -h flag
2. Test with -c flag with config file path
3. Test with -c flag config file name

Check -h flag

> sudo sh ./CreatePersistentImg.sh -h
Usage:  sudo ./CreatePersistentImg.sh [ -s size ] [ -t fstype ] [ -l LABEL ] [ -c CFG ] [ -e ]
  OPTION: (optional)
   -s size in MB, default is 1024
   -t filesystem type, default is ext4  ext2/ext3/ext4/xfs are supported now
   -l label, default is casper-rw
   -c specify the path to the config file what to be put into the persistence file.
      If not exist, config file name is the path basename and file content is "/ union"
   -o outputfile name, default is persistence.dat
   -e enable encryption, disabled by default (only few distros support this)

Check -c flag

Testplan:

1. Enable commands echo with set -x
2. Create a persistent with custom persistence.conf
3. Mount the persistent as loop
4. Compare source persistence.conf and the one from the mounted loop

Test with -c filepath

> sudo sh ./CreatePersistentImg.sh -o ../pers.dat -c ../persistence.conf
...
+ [ -n ../persistence.conf ]
+ [ -d ./persist_tmp_mnt ]
+ mkdir ./persist_tmp_mnt
+ mount /dev/loop0 ./persist_tmp_mnt
+ config_name=persistence.conf
+ [ -f ../persistence.conf ]
+ cat ../persistence.conf
+ sync
+ umount ./persist_tmp_mnt
+ rm -rf ./persist_tmp_mnt
+ [ ! -z  ]
+ losetup -d /dev/loop0

> sudo mount -o loop ../pers.dat /tmp/pers/

> diff -s ../persistence.conf /tmp/pers/persistence.conf
Files ../persistence.conf and /tmp/pers/persistence.conf are identical

Test with -c filename

> sudo sh ./CreatePersistentImg.sh -o ../pers.dat -c persistence.conf
...
+ [ -n persistence.conf ]
+ [ -d ./persist_tmp_mnt ]
+ mkdir ./persist_tmp_mnt
+ mount /dev/loop0 ./persist_tmp_mnt
+ config_name=persistence.conf
+ [ -f persistence.conf ]
+ echo / union
+ sync
+ umount ./persist_tmp_mnt
+ rm -rf ./persist_tmp_mnt
+ [ ! -z  ]
+ losetup -d /dev/loop0

> sudo mount -o loop ../pers.dat /tmp/pers/

> diff -s <(echo '/ union') <(cat /tmp/pers/persistence.conf)
Files /dev/fd/63 and /dev/fd/62 are identical

> cat /tmp/pers/persistence.conf
/ union