Activate new swap after repartitioning

Activate newly created swap partition for low-memory devices. For my phytech pcm051 board this avoids OMM killer appearance on first boot.

```
[1416700816.865796] Creating swap space on /dev/mmcblk0p3
[1416700817.053569] Activating swap space on /dev/mmcblk0p3
[   35.432725] Adding 505856k swap on /dev/mmcblk0p3.  Priority:-1 extents:1 across:505856k SSFS
```