This change makes it so that the ARM image creation script uses DNF
instead of urpmi. This affords several advantages:
1. Mageia images can be created from non-Mageia systems
2. Mageia images can be created from within Mock
3. ARM images can be created from x86_64 hosts
4. Image building can rely on pre-installed repo configurations
This also brings us in line with the Docker image creation process,
which also uses DNF.
