Check for boot flag before possibly creating FAT16 on the partition

Check for boot flag before possibly creating FAT16 on the partition

Checks should be performed before actually modifying the USB device,
if the user specified `--fat16` then the boot flag should be checked
*before* the FAT filesystem gets created.

check_for_fat(device) is called in handle_vfat(device) (if device is
present and not a directory and if option 'syslinux' (default) is set)
so it should be safe to remove check_for_fat(device) call in
install_grml().

The same applies to check_boot_flag() which should happen before
actually creating the FAT filesystem (in handle_vfat()).

Closes: grml/grml2usb#30