Do not create device-mapper mapping if no subpartitions
This causes initramfs to skip calling 'kpartx' when pmOS has been installed on an external disk (e.g. sdcard).
Tested on my N900 and works great.
Note that this will cause the 'resize' stuff to be skipped over, but it's not needed because the call to parted for creating the root partition already instructs parted to use 100% of the available space, and since it's working on a real device, there's no need to resize in initramfs :)
fixes #774 (closed)