fontcache.bbclass: add fontconfig-utils runtime dependency

In an image recipe, you can get a warning, for example:
WARNING: The postinstall intercept hook 'update_font_cache' failed (exit code: 1)! See log for details!
WARNING: The postinstalls for the following packages will be postponed for first boot: ttf-dejavu-sans-mono

(because /usr/bin/fc-cache is missing)

In OE-core, rdepend is correctly done in each recipe:
- ttf-fonts/liberation-fonts
- ttf-fonts/ttf-bitstream-vera

In meta-OE, rdepend is not done but packagegroup-fonts-truetype.bb includes fontconfig-utils:
- ttf-fonts/ttf-arphic-uming
- ttf-fonts/ttf-dejavu
- ttf-fonts/ttf-droid
- ttf-fonts/ttf-gentium
- ttf-fonts/ttf-hunkyfonts
- ttf-fonts/ttf-inconsolata
- ttf-fonts/ttf-liberation
- ttf-fonts/ttf-mplus
- ttf-fonts/ttf-sazanami
- ttf-fonts/ttf-ubuntu-font-family
- ttf-fonts/ttf-wqy-zenhei

(From OE-Core rev: 306335b13fedc18cf03da1c2a68e97c01eb59075)

Signed-off-by: Matthieu Crapet <Matthieu.Crapet@ingenico.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/fontcache.bbclass

@@ -7,6 +7,7 @@ DEPENDS += "qemu-native"
 inherit qemu
 
 FONT_PACKAGES ??= "${PN}"
+FONT_EXTRA_RDEPENDS ?= "fontconfig-utils"
 
 fontcache_common() {
 if [ "x$D" != "x" ] ; then
@@ -19,8 +20,11 @@ fi
 
 python populate_packages_append() {
     font_pkgs = d.getVar('FONT_PACKAGES', True).split()
+    deps = d.getVar("FONT_EXTRA_RDEPENDS", True)
 
     for pkg in font_pkgs:
+        if deps: d.appendVar('RDEPENDS_' + pkg, ' '+deps)
+
         bb.note("adding fonts postinst and postrm scripts to %s" % pkg)
         postinst = d.getVar('pkg_postinst_%s' % pkg, True) or d.getVar('pkg_postinst', True)
         if not postinst: