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

Change-Id: Ib5e8b8eca3cfba1aa3b3c422be5de8af8af3102c