repart: Use 4096 as the fallback sector size for verity/luks/filesystems

repart: Use 4096 as the fallback sector size for verity/luks/filesystems

When we don't know the sector size of the actual block device, because
we're building an image in a loopback file and no sector size was specified
explicitly, let's use 4096 as the sector size for filesystems, verity and
LUKS. This should be the most compatible option, since 4096 will also work
on devices with sector size 512 or 2048.

For the actual GPT partition table size, we stick with 512 as the default
value since UEFI firmware and the kernel will only try to read the GPT
partition table from the first LBA on the device and the sector size for
most devices is still 512. It can also be trivially modified when copying
the image to another device using --copy-from + --sector-size.