Menderize - Image converter UI

In MEN-1812, we create CLI tools to convert images to support Mender.
In order to improve ease of use, in particular for prototyping and demo cases, it would be nice to have a UI on top of this CLI.
This UI will run locally (e.g. on a laptop) because the disk image files can be very large and it is much faster to process them locally.

The purpose of this UI is to cover more low-scale / prototyping cases, where the user has a "golden SD card" that contains the stock distribution with some run-time modifications (e.g. Raspbian with some additional packages installed). Updates are created by booting into this SD card, making modifications, powering off the device and copying it into a laptop for conversion and distribution.

Acceptance criteria

• Given a SD card (plugged in) and Mender binaries, a "Menderized" disk image (sdimg) file can be generated (for writing to other SD cards; it contains Mender and all it needs to run)
• Given a SD card (plugged in) and Mender binaries, a Mender Artifact (.mender) file can be generated (for uploading to- and deployment from the Mender server)
• It is possible to input Server URL, tenant token, Release name and output SD card size