Drop old bootflag detection + be more verbose about its execution

Our custom boot flag detection isn't reliable, so there's no
point in using it, instead let's rely on python3-parted only.

While at it, rename "bootflag" into "boot flag" in human readable
strings.

While at it, also be more verbose about the execution (esp. the
"sync" step takes a while and might look like something is
hanging).

Thanks: Darshaka Pathirana for bug report and debugging
Closes: grml/grml2usb#19