avb: Allow system partition to be absent

On devices with dynamic partitions,'system' partition might not exist.
Therefore, we should not add root=$(ANDROID_SYSTEM_PARTUUID) in kernel
cmdline and resolve it for devices without system partition.

Bug: 119551429
Test: m libavb_host_unittest &&
      ../../out/host/linux-x86/nativetest64/libavb_host_unittest/libavb_host_unittest

Git-repo: https://source.codeaurora.org/quic/la/platform/external/avb
Git-Commit: 5abd6bc2578968d24406d834471adfd995a0c2e9

Change-Id: I99d56c103a2ca562574db38bfce22a61a0088484