Changed preference for selecting package definition files, so that it takes the one from our tree in preference to the one in the package itself. This means that we can just delete items from our tree when we're happy to switch to using the one in the package (as this will vary from package to package). Added sorting of the attributes when outputting the system definition, to match the de facto standard in the system definition we started with.

TRUST LEVEL 5 MACHINE sym-build01.*
TRUST LEVEL 5 MACHINE sym-build02.*
TRUST LEVEL 5 MACHINE lon-engbuild87.*
TRUST LEVEL 5 MACHINE lon-engbuild89.*