make: fix '--plugins=' option not being respected in all cases

If the list of plugins given to configure did not include
a subdir-based plugin, then all subdir-based plugins would
be built (since SELECTED_PLUGINS_SUBDIRS is empty then).

To prevent this issue use an explicit value ('DEFAULT') to
signal that we want to build the standard set of plugins.

Change-Id: I42d9dc8d754ed9ac1cd4b5c62c0a106ca3dfcd91

tools/configure

@@ -4848,6 +4848,10 @@ if [ -n "$ARG_PLUGINS" ]; then
              echo "Warning: Plugin '$p' found neither as file nor directory in $chk_plugin_dir. Use ':' as the separator."
         fi
     done
+else
+    # Build the default set of plugins
+    selected_src="DEFAULT"
+    selected_subdirs="DEFAULT"
 fi
 
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