Restore augmentation sorting semantics

Restore augmentation sorting semantics

Augmentation sort we are performing to order augmentations has another,
undocumented, impact. It provides stability of generated code towards
reformats of the YANG source, so that changing the order of augment
statements (by itself) does not change the naming of generated fields.

The simple comparator failed to take this effect, leading to an
incompatible change of assignments which is extremely hard to untangle.
Fix this by restoring previous sorting behaviour.

Change-Id: I7164e6e06d47fcded3261752a8a0f587afc70274
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>