Add function to extract timestamp from boot image

Also add toybox to otatools list.

Bug: 169169031
Test: build OTA
Change-Id: Ib14dbf46a8385ccf2a9c5a9c6f4e7fa9399cf0ba
diff --git a/tools/releasetools/common.py b/tools/releasetools/common.py
index f5dfbec..f256900 100644
--- a/tools/releasetools/common.py
+++ b/tools/releasetools/common.py
@@ -814,6 +814,15 @@
     props._LoadBuildProp(data)
     return props
 
+  @staticmethod
+  def FromBuildPropFile(name, build_prop_file):
+    """Constructs an instance from a build prop file."""
+
+    props = PartitionBuildProps("unknown", name)
+    with open(build_prop_file) as f:
+      props._LoadBuildProp(f.read())
+    return props
+
   def _LoadBuildProp(self, data):
     for line in data.split('\n'):
       line = line.strip()