commands: add --update option to spack list

- Add a `--update FILE` option to `spack list`
- Output is written to the file only if any package is newer than the file
- Simplify the code in docs/conf.py using this new option

lib/spack/docs/conf.py

@@ -54,8 +54,8 @@
 os.environ['COLUMNS'] = '120'
 
 # Generate full package list if needed
-subprocess.Popen(
-    ['spack', 'list', '--format=html', '--update=package_list.html'])
+subprocess.call([
+    'spack', 'list', '--format=html', '--update=package_list.html'])
 
 # Generate a command index if an update is needed
 subprocess.call([