component/firefox: generate a fallback $FONTCONFIG_FILE

Since e5687602 it's instance
responsability to generate a fonts.conf if they need fonts, but we
realized to late that if there is no /etc/fonts/fonts.conf on the
machine firefox does not start.

In recent ERP5 software release, we have a $FONTCONFIG_FILE, but on
older versions we don't.

To keep compatibility with old ERP5 software release, we generate a
default FONTCONFIG_FILE that will be used as fallback when
$FONTCONFIG_FILE environment variable is not set.

component/firefox/buildout.cfg

@@ -40,6 +40,9 @@ install =
   cd {}
   export LD_LIBRARY_PATH=$PWD:{}
   export PATH={}:$PATH
+  # BBB use a default fonts.conf for compatibility, but it's software instance
+  # responsability to build a fonts.conf with the fonts they want.
+  [ -z $FONTCONFIG_FILE ] && export FONTCONFIG_FILE=${firefox-default-fonts-conf:rendered}
   exec ./firefox "$@"
   """.format(
     firefox,
@@ -68,6 +71,21 @@ part = ${firefox-52:location}
 wrapper-name = firefox-51
 part = ${firefox-51:location}
 
+[firefox-default-fonts-conf]
+recipe = slapos.recipe.template:jinja2
+template = ${template-fonts-conf:output}
+rendered = ${buildout:parts-directory}/${:_buildout_section_name_}/fonts.conf
+context =
+  key cachedir :cache-dir
+  key fonts :fonts
+  key includes :includes
+fonts =
+  ${ipaex-fonts:location}
+  ${liberation-fonts:location}
+includes =
+  ${fontconfig:location}/etc/fonts/conf.d
+cache-dir =
+  ~/.fontconfig-firefox/
 
 [firefox]
 # The default installed firefox version when installing firefox-wrapper.