Hidden dependency on kpartx under linux

[pelrun]

If kpartx isn't installed then a non-useful "undefined: not able to write image undefined" error is thrown after the image is written but before the configuration is.

Needs a mention in the installation instructions and a runtime check to see if it's present.

[aaronpalermo]

My error did mention kpartx, but only after several minutes of writing the SD card, then errored out. Checking for kpartx prior to writing the SD card would definitely be helpful.

[davidferguson]

Good idea, I'll be adding this in to the next version.

[00p513-dev]

I got this error on the version 2 pre-release. Maybe this should be added?