Use pyparted to check for bootflag [Closes: issue1248]

The approach to check the 7th field in the MBR for the
bootable flag being set isn't enough. This fails to detect
bootable partitions if they aren't the first primary ones.

pyparted provides a way to check whether the specified
partition really has the boot flag set, so let's use this
approach, iff pyparted is available.

The check can also be completely skipped using the
--skip-bootflag parameter.

Took the get_partition_for_path function from Evgeni Golov's
https://github.com/evgeni/grml2usb/tree/pyparted with his
permission.