app: config: Add support for appending to the config string

In some cases, and specifically in the manifest.group-filter and
manifest.project-filter options, it is sometimes useful to be able to
append to a value instead of replacing it completely.

For example, assuming one wants to add to an existing group filter,
without this patch the user needs to do:

(assuming the group filter is currently +unstable,-optional, and the
user wants to add +extras).
> west config manifest.group-filter
> west config manifest.group-filter +unstable,-optional,+extras

With this patch instead:

> west config -a manifest.group-filter ,+extras

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>

src/west/app/config.py

@@ -48,6 +48,9 @@
 To set a value for <name>, type:
     west config <name> <value>
 
+To append to a value for <name>, type:
+    west config -a <name> <value>
+
 To list all options and their values:
     west config -l
 
@@ -111,7 +114,9 @@ def do_add_parser(self, parser_adder):
                             help='delete an option in one config file')
         parser.add_argument('-D', '--delete-all', action='store_true',
                             help="delete an option everywhere it's set")
-
+        parser.add_argument('-a', '--append', action='store_true',
+                            help='append to an existing value')
+
         group = parser.add_argument_group(
             'configuration file to use (give at most one)')
         group.add_argument('--system', dest='configfile', nargs=0, action=Once,
@@ -135,11 +140,18 @@ def do_run(self, args, user_args):
                 self.parser.error('-l cannot be combined with name argument')
             elif delete:
                 self.parser.error('-l cannot be combined with -d or -D')
+            elif args.append:
+                self.parser.error('-l cannot be combined with -a')
         elif not args.name:
             self.parser.error('missing argument name '
                               '(to list all options and values, use -l)')
-        elif args.delete and args.delete_all:
-            self.parser.error('-d cannot be combined with -D')
+        elif args.delete:
+            if args.delete_all:
+                self.parser.error('-d cannot be combined with -D')
+            elif args.append:
+                self.parser.error('-d cannot be combined with -a')
+        elif args.value is None and args.append:
+            self.parser.error('-a requires a value')
 
         if args.list:
             self.list(args)
@@ -194,8 +206,14 @@ def read(self, args):
     def write(self, args):
         self.check_config(args.name)
         what = args.configfile or LOCAL
+        if args.append:
+            value = self.config.get(args.name,
+                                    configfile=args.configfile or ALL) or ''
+            value += args.value
+        else:
+            value = args.value
         try:
-            self.config.set(args.name, args.value, configfile=what)
+            self.config.set(args.name, value, configfile=what)
         except PermissionError as pe:
             self._perm_error(pe, what, args.name)