remaster_iso_functions: add trap to umount chroot bind mounts

[sundarnagarajan]

When running for chroot target, a number of dirs are mounted (with --bind option). If the user presses CTRL-C or the program abends for any reason during this phase,:

• Those directories remain mounted
• The extract dir cannot be removed because of those mounts

Solution:

• Setup a TRAP before entering the chroot
• Do a umount -f in the trap