Add support for defining variables from other variables or basic expression

Using this we can now define variables in the variable section using
basic expression that are evaluated as f-string.
For example, you can use the syntax:

```
"mpp-vars": {
   "rootfs_uuid": {"mpp-format-string": "{__import__('uuid').uuid1()}"},
   "bootfs_uuid": "156f0420-627b-4151-ae6f-fda298097515"
},
```

This will automatically call uuid.uuid1() for rootfs_uuid, thus allowing
to dynamically set the uuid for the rootfs variable.
This variable being able to be overridden via the -D argument of the
osbuild-mpp tool.

In addition, you can also define variable based on variables defined
above, for example:

```
"mpp-vars": {
   "rootfs_size": 4294967296,
   "homefs_size": {"mpp-format-string": "{rootfs_size}"}
},
```

For context, we're in particular interested in this for the uuid
generation, as we have had issues with xfs failing to mount a filesystem
if the uuid is already mounted. In particular, if we built e.g. a
raspberry pi image using osbuild and then we use that raspberry pi for building
the same manifest (typically a later version of it) the osbuild fails in the
loopback mount case. Being able to easily support reproducible fixed-uuid
images, as well as ones with a real unique uuid makes a lot of sense.

This commit was created with a lot of inputs from Alexander Larsson.

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

tools/osbuild-mpp

@@ -555,7 +555,10 @@ class ManifestFile:
         if not variables:
             return
 
-        self.vars.update(variables)
+        for k, v in variables.items():
+            fakeroot = [v]
+            self._process_format(fakeroot)
+            self.vars[k] = fakeroot[0]
         self.substitute_vars(self.vars)
 
     def set_vars(self, args):