Output the amended repart definitions to a well-known directory in
$TMPDIR instead of using a temporary directory with a random directory
name.
The output file `repart-output.json` also contains the full path to the
repart definition file used to create the partition. As
`amend-repart-definitions.py` uses `tempfile.mkdtemp`, this introduces
an impurity:
```json
{
"type" : "root-x86-64",
"label" : "rootfs",
"uuid" : "f2fa2e49-e443-45d2-a2e2-c3754cab6363",
"file" : "/build/tmppjo7kv5o/rootfs.conf",
"node" : "image.raw2",
"offset" : 135266304,
"old_size" : 0,
"raw_size" : 1651101696,
"old_padding" : 0,
"raw_padding" : 0,
"activity" : "create",
}
```
This commit changes the parent directory of the amended repart
definitions to `/build/amended-repart.d/`.
