bundle.bbclass: fix variable expansion of RAUC_META_* varflags

Variables assigned to `RAUC_META_<section>[varflag]` are not expanded
anymore. This is caused by `getVarFlags` not expanding variables by
default, revert to using `getVarFlag` which does expand variables by
default.

Fixes: #335
Fixes: ffba61f ("bundle.bbclass: simplify varflag usage")
Signed-off-by: Ludovic Jozeau <ludovic.jozeau@scle.fr>
Signed-off-by: Jan Luebbe <jlu@pengutronix.de>
(cherry picked from commit 8fd3907)
Signed-off-by: Enrico Jörns <ejo@pengutronix.de>

classes/bundle.bbclass

@@ -307,7 +307,8 @@ def write_manifest(d):
 
     for meta_section in (d.getVar('RAUC_META_SECTIONS') or "").split():
         manifest.write("[meta.%s]\n" % meta_section)
-        for meta_key, meta_value in d.getVarFlags('RAUC_META_%s' % meta_section).items():
+        for meta_key in d.getVarFlags('RAUC_META_%s' % meta_section):
+            meta_value = d.getVarFlag('RAUC_META_%s' % meta_section, meta_key)
             manifest.write("%s=%s\n" % (meta_key, meta_value))
         manifest.write("\n");