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.

Closes: #30

grml2usb

@@ -1875,10 +1875,11 @@ def install_grml(mountpoint, device):
         register_tmpfile(device_mountpoint)
         remove_device_mountpoint = True
         try:
-            check_for_fat(device)
             if not options.skipbootflag:
                 check_boot_flag(device)
 
+            check_for_fat(device)
+
             set_rw(device)
             mount(device, device_mountpoint, ["-o", "utf8,iocharset=iso8859-1"])
         except CriticalException: