Load boot build props to info_dict properly.

Add boot to PARTITION_WITH_BUILD_PROP. Let PartitionBuildProps handle
boot partitions by extracting build.prop file.

Test: generate OTA and call
      `payload_info.py payload.bin`, check that boot image timestamp is
      set
Bug: 162623577

Change-Id: I71955e4ee494ffbb5d111c3d02a129c0f879b6d1