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 +48,11 @@
 To set a value for <name>, type:
     west config <name> <value>
 
+To append to a value for <name>, type:
+    west config -a <name> <value>
+A value must exist in the selected configuration file in order to be able
+to append to it.
+
 To list all options and their values:
     west config -l
 
@@ -64,7 +69,7 @@
 
 CONFIG_EPILOG = '''\
 If the configuration file to use is not set, reads use all three in
-precedence order, and writes use the local file.'''
+precedence order, and writes (including appends) use the local file.'''
 
 ALL = ConfigFile.ALL
 SYSTEM = ConfigFile.SYSTEM
@@ -88,12 +93,19 @@ def do_add_parser(self, parser_adder):
             description=self.description,
             epilog=CONFIG_EPILOG)
 
-        parser.add_argument('-l', '--list', action='store_true',
+        group = parser.add_argument_group(
+            "action to perform (give at most one)"
+        ).add_mutually_exclusive_group()
+
+
+        group.add_argument('-l', '--list', action='store_true',
                             help='list all options and their values')
-        parser.add_argument('-d', '--delete', action='store_true',
+        group.add_argument('-d', '--delete', action='store_true',
                             help='delete an option in one config file')
-        parser.add_argument('-D', '--delete-all', action='store_true',
+        group.add_argument('-D', '--delete-all', action='store_true',
                             help="delete an option everywhere it's set")
+        group.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)"
@@ -121,20 +133,20 @@ def do_run(self, args, user_args):
         if args.list:
             if args.name:
                 self.parser.error('-l cannot be combined with name argument')
-            elif delete:
-                self.parser.error('-l cannot be combined with -d or -D')
         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.value is None and args.append:
+            self.parser.error('-a requires a value')
 
         if args.list:
             self.list(args)
         elif delete:
             self.delete(args)
         elif args.value is None:
             self.read(args)
+        elif args.append:
+            self.append(args)
         else:
             self.write(args)
 
@@ -179,6 +191,16 @@ def read(self, args):
             self.dbg(f'{args.name} is unset')
             raise CommandError(returncode=1)
 
+    def append(self, args):
+        self.check_config(args.name)
+        what = args.configfile or LOCAL
+        value = self.config.get(args.name, configfile=what)
+        if value is None:
+            self.die(f'option {args.name} not found in {what.name.lower()} '
+                     'configuration file')
+        args.value = value + args.value
+        self.write(args)
+
     def write(self, args):
         self.check_config(args.name)
         what = args.configfile or LOCAL