not all devices/distros have mknod, include busybox's (#56)

It seems that the HTC One does not include mknod, luckily busybox
provides it.  Thanks to peterosterlund2 for this fix.
when creating a loop dev, manually specify permissions (closes #56)

"The umask is 000 so the loop device permission needs to be
explicitly set to 600" -peterosterlund2  (thanks for the patch!)