Skip vbmeta_system if it doesn't exist

Test: validate_target_files bonito-target_files-7582772.zip
Change-Id: I2da4f5a7897b1ef155e17441a8f5b97c0716d601

tools/releasetools/common.py

@@ -459,6 +459,10 @@ class BuildInfo(object):
   def oem_props(self):
     return self._oem_props
 
+  @property
+  def avb_enabled(self):
+    return self.get("avb_enable") == "true"
+
   def __getitem__(self, key):
     return self.info_dict[key]