classes: bundle: use getVarFlag instead .get

Make use of .getVarFlags instead of the simple .get to allow bitbake
level variable expansion.

This is useful especially when using bundle variants [1], on a
devicetree based board, where the compatible string - the slot
'name' -  would be constructed from another variable found e.g. in the
machine.conf

Link: https://rauc.readthedocs.io/en/latest/advanced.html#sec-variants
Signed-off-by: Johannes Schneider <[email protected]>

classes/bundle.bbclass

@@ -231,17 +231,17 @@ def write_manifest(d):
     for slot in (d.getVar('RAUC_BUNDLE_SLOTS') or "").split():
         slotflags = d.getVarFlags('RAUC_SLOT_%s' % slot)
         if slotflags and 'name' in slotflags:
-            slotname = slotflags.get('name')
+            slotname = d.getVarFlag('RAUC_SLOT_%s' % slot, 'name')
         else:
             slotname = slot
         manifest.write('[image.%s]\n' % slotname)
         if slotflags and 'type' in slotflags:
-            imgtype = slotflags.get('type')
+            imgtype = d.getVarFlag('RAUC_SLOT_%s' % slot, 'type')
         else:
             imgtype = 'image'
 
         if slotflags and 'fstype' in slotflags:
-            img_fstype = slotflags.get('fstype')
+            img_fstype = d.getVarFlag('RAUC_SLOT_%s' % slot, 'fstype')
         else:
             img_fstype = d.getVar('RAUC_IMAGE_FSTYPE')