It seems some tools needed by devtool are not available in a fresh tree, and are not built automatically by devtool. "devtool modify mdadm" fails in a clean build folder. The problem is that xz is not on the path. Running 'bitbake xz-native' makes xz available and 'devtool modify mdadm' succeeds. I'm guessing other packages have other prerequisites that may be missing depending on the fetcher and any downloaded file types. Should devtool auto-build the fetcher dependencies, or ask the user to do so? Building up until the unpack task of the target recipe should work for getting any prerequisites built.
This was on poky master, don't know if it is a problem on pyro or earlier.
This is a known bug unfortunately - I'm still trying to figure out how to rework the code so that this can be fixed. *** This bug has been marked as a duplicate of bug 11198 ***