Make GenericMakeStep not immutable

GenericMakeStep is currently immutable. This is quite annoying since
it's possible to create GenericMakeStep but it's not possible to remove
them.

Fix: make GenericMakeStep not immutable

Merge-request: 117
Reviewed-by: Thorbjørn Lindeijer <thorbjorn.lindeijer@nokia.com>
Reviewed-by: dt
Reviewed-by: Tobias Hunger