Make iso image 4GB+

[pavroo]

Due to limitation of 'iso9660' standard which is use by 'xorriso', sparkybackup tool can create iso images up to 3.99GB.

The 'sparky-backup-core' 20200221 uploaded to repo today has a new option (experimental) to build bigger iso image than 4GB using 'genisoimage' with 'udf' file system instead of 'iso9660'.

To get it working with 'genisoimage', do:
1. upgrade 'sparky-backup-core' >= 20200221
2. install 'genisoimage'
3. remove 'xorriso'

I tested it on small iso 1GB only, works fine on bios and efi machines, but did not try to build iso image 4GB+.

Let me know if you find any problem, please.