run: concatenate 'install_config' args

This patch enables one to supply a list of arguments to
'install_config'. The arguments appear concatenated in the resulting
config file. This is useful for embedding dynamically generated content
into an otherwise static configuration. For example, in the following
config, the 'pkg' attribute of the '<query>' config node is defined by
the return value of the 'query_pkg' function:

  install_config {
  <config>
    ...
    <start name="depot_query">
      ...
      <config>
        <query pkg="genodelabs/pkg/} [query_pkg] {"/>
      </config>
    </start>
  </config>}

tool/run/run

@@ -108,9 +108,9 @@ proc check_xml_syntax {xml_file} {
 ##
 # Install content of specfied variable as init config file
 #
-proc install_config {config} {
+proc install_config { args } {
 	set fh [open "[run_dir]/genode/config" "WRONLY CREAT TRUNC"]
-	puts $fh $config
+	puts $fh [join $args {}]
 	close $fh
 
 	check_xml_syntax [run_dir]/genode/config