mender-convert output image naming convention

Currently the output images have the suffix "...sdimg.gz". However, the input images always have the .img suffix (plus some compression format), so the ".sdimg" suffix intends to show that "this is a .img image with Mender support".

But .sdimg is a very non-standard format, especially for Debian family images. Users will likely wonder what type of image this is, when it really is the same type as a .img image. At the same time we should make it more explicit which images have been integrated with Mender and which have not.

Rather than the "...sdimg.gz" suffix, we should use a '...-mender.img.gz" suffix. For example:

For input image:

sr-imx6-debian-buster-20190925-cli.img

The output image is currently:

sr-imx6-debian-buster-20190925-cli.sdimg.gz

But it should be:

sr-imx6-debian-buster-20190925-cli-mender.img.gz

Acceptance criteria

• Output images have the "...-mender.img.gz" suffix