Partitions that use vbmeta_system can be found by looking at
avb flag in fstab from boot ramdisk.
--flags 2 makes the verification function of avb always return a
positive result.
--set_hashtree_disabled_flag builds the vbmeta images with the
HASHTREE_DISABLED bit set and as a result they don't need to manually
disable dm-verity via e.g. 'adb disable-verity'.
The rest of the configuration is taken from
https://source.android.com/devices/tech/ota/dynamic_partitions/implement#avb-configuration-changes
Change-Id: I381feef8f6fefc8449ca50d85d704b67bcc8a77e